Gilligan's Island/WMG

Each Castaway is one of the Seven Deadly Sins, and Gilligan is Satan.
Ginger is Lust (obviously). Mary Ann is Envy: she is jealous of Ginger's beauty. The Professor is Pride: he's proud of being an annoying know-it-all. Mr. Howell is Greed (obviously). Mrs. Howell represents Sloth: she has never lifted a finger to help on any of the escape plans, except the Honeybees one. The Skipper represents two sins: Gluttony, and Wrath (for how he treats Gilligan). This leaves Gilligan. Gilligan is the person who put them there. He prevents them from leaving by foiling all of their escape plots. Also, it is his island. Therefore, Gilligan is Satan. Crazy? He does wear red in every episode. (Adapted from this site.)
 * The Professor's summary of their experiences in Rescue From Gilligan's Island all but hangs a lampshade on this theory.
 * The reason Skipper is two sins instead of, say, Gilligan being Gluttony, is that Wrath was the original first mate, and the Skipper ate him and absorbed his wrath-power. Gilligan (Satan) replaced him, which is why they don't Just Eat Gilligan.
 * So that means Gilligan's Island is Hell??? Somehow I never pictured Hell as a tropic isle...
 * Or they're n purgatory for committing said sins.

Each Castaway, including Gilligan, is one of the Seven Deadly Sins.
Pairings are the same as the previous entry, except the Skipper is Anger only, and Gilligan is Gluttony. Really, if they wanted to have each of the Deadly Sins plus Satan, how hard would it have been to have eight castaways? Having one character represent two sins is aesthetically unsatisfying, and likely invented by unimaginative people who confuse obesity with gluttony. The Skipper may be fat, but he doesn't appear to eat any more than any of the others. Gilligan, on the other hand, is often portrayed as constantly eating whatever he can get his hands on.
 * No, Mrs. Howell = gluttony and Gilligan = sloth. Gilligan as sloth is easy to explain; Mrs. Howell is gluttony because she constantly surrounds herself with her possessions in a hedonistic way.
 * Partly confirmed, according to Inside Gilligan's Island! Google Books shows a bit of the book, and it does in fact say that each castaway represents one sin. The pages explaining which is which aren't present, but someone at Cracked read it and wrote about it, giving four sins--Gilligan's Sloth, the Professor's Pride, Ginger's Lust, and the Skipper's Wrath.
 * According to this link, the other three sins are listed as Mary Ann as Envy, Thurston Howell III as Avarice/Greed, and Lovey Howell as Gluttony.

The Castaways are the subjects of an experiment run by The Professor.
Nobody could be that polycompetent at so many sciences and Bamboo Technology and still be unable to fix a 3-foot hole in a boat. Whether the Professor arranged for the wreck or not is unknown, but since arriving on the island, he has taken advantage of the controlled environment to perform fiendishly subtle experiments on the others. When he doesn't directly sabotage escape attempts himself, he uses Gilligan -- who is known to be dangerously susceptible to hypnotic suggestion -- as his fall guy.

The Castaways are the subjects of an experiment run by Gilligan.
This is why Gilligan sabotages all attempts to get off the island but is immensely helpful on the rare occasions when the problem is coming from the island. He's using Obfuscating Stupidity so heavily that his experiment is almost double-blind.

The Castaways are the subjects of an experiment run by a Government Conspiracy.
The Professor can't fix the hole in the boat because the Mind Control won't let him. As in the above scenario, Gilligan is under further mind control to sabotage any escape attempts. Further details are available here(pdf).

The Castaways (except The Professor) are the subjects of an experiment by the Dharma Initiative.
The Professor is an overseer working for Dharma. The characters of Lost are another version of the same experiment; by extension, the Dharma Initiaive are still very much organized. The purge was faked to draw suspicion away from them.
 * Then Gilligan must be the will of the island personified. Why else would he sabotage their attempts to leave the damn place?
 * And all the guest stars are either Dharma or Others. It was filmed long enough ago for it to be either/or.

The castaways did Just Eat Gilligan.
The entire series is a round-robin story they're telling to ease their tortured consciences as they sail back home.

The castaways are all a figment of Gilligan's imagination.
After being marooned on the island during a lone boating trip, Gilligan went insane and invented the entire story of the show. The other castaways are a combination of personality with simple, broad concepts (like the Seven Deadly Sins); they never think of eating Gilligan because the entire show takes place in Gilligan's mind. Their attempts to escape the island are sabotaged each time because Gilligan himself, mad though he is, acknowledges that he will never escape.
 * Or, the castaways were real, on the boat. Gilligan only met them briefly, so only knew them as charicatures. They died in the shipwreck and Gilligan hallucinates them because they were the last people he ever interacted with.

Gilligan is a former gang hitman who turned on his boss, killed him, and stole a bunch of money.
He probably thought he could get away with it, too. But when he realized they were closing in on him, he got the job on the ship, caused the wreck, and has been sabotaging all the repair efforts ever since.
 * Debunked by question: how did Gilligan control the weather?

The Castaways are eating Gilligan, and the entire show is his Dying Dream.
That's why the episodes change over time and become more vivid (going from black and white to color), sillier and more surreal, and (just like movies) lengthier, and still always lead back to the island. That's why Gilligan eats so much-- he associates death with food imagery, and his constant eating represents a sublimated desire for revenge. His sabotaging every "escape attempt" is also driven by revenge fantasies. Why else would they keep running into cannibals? And several episodes have revolved around eating things, or having things in one's mouth, that turned out to have negative consequences-- radioactive vegetables, mind-reading seeds that brought out everyone's darkest secrets, poison mushrooms, explosive tooth fillings...

There never was a show called "Gilligan's Island."
The subject of this entry is just the longest continuous "Twilight Zone" episode ever produced.

There was no professor
The Professor was killed early on, and the other castaways create a shared delusion to avoid dealing with his death. Think about it:
 * 1) No one with a science background would come up with some of the dumb ideas he has come up with.
 * He still uses more scientific terminology correctly than their imaginations could produce
 * 1) The Professor has his own hut, which is rarely seen and which seems to be just storage for random junk the few times it is seen.
 * 2) He is just "the Professor."

There was a professor -- but he was a professor of psychology, not engineering.
The rest of the crew slayed him before he could realize that they're all homicidal madmen. The other islanders then used him as an increasingly grotesque rotting meat puppet to come up with an idea each week. This is why he had some reasonable ideas in the first couple of episodes but tries increasingly impossible things as time goes on. (This also adds the disturbing implication that Ginger and, in one episode, Maryanne tried to go necrophile in the second season.)

Gilligan deliberately sabotaged all attempts to get off the island.
Basically, Gilligan is a case of Obfuscating Stupidity. Gilligan is, in fact, a highly trained government agent tasked with keeping a small segment of the population alive in an isolated environment to survive the imminent nuclear war between the U.S. and Soviet Union. Once the war is over and everyone else in the world is dead, Gilligan will reveal all this to his fellow islanders and they will restart civilization.

They aren't really stranded on an uncharted desert isle.
It's a peninsula, and they all fail Geography forever.
 * Win!
 * No one ever bothered to map out the entire island, although some of them have talked about "the other side of the island," no one ever bothered to go to the beach, pick a direction and start walking?

The Professor doesn't want to get off the island.
Think about it. He's the only eligible man stranded on an island with Ginger and Mary Ann. Would you want to leave?
 * So Gilligan and the Skipper are gay?
 * This theory has been advanced by the actor who played The Professor in interviews - the logic being that the professor is trapped with two hot women on an island where the only competition is an old, married guy, a fat guy constantly throwing a temper tantrum, and an idiot who causes no less than half of everyone's misfortunes. He'd practically win by default.
 * Alternately, the Professor doesn't want to leave the island via a repaired Minnow, because he realizes that they have no idea where they are in the vast Pacific -- they got blown to the island in a storm, remember? -- and would probably wind up dying of thirst in the open sea rather than being rescued. Rather than have the others outvote him and decide to risk it because they're sick of being stranded, he pretends the boat is beyond repair so they'll remain safely on land, where there's food and fresh water and hot chicks.

Gilligan is a reincarnation of an actual headhunter demigod... and he may not be alone.
He is unaware of the many ways in which his powers regularly manifest because in this particular lifetime he happens to be retarded. However, this is not only why he is so hard to kill or injure, but also why he seems to be mystically drawn towards bizaar accidents that trap people in cannibal/headhunter territory. Incidentally, the reason that there continue to be cannibals and headhunters so close to Hawaii is that to each generation, there is a a chosen, blah blah blah blah blah. perhaps the last one was a cartographer.

Sub-hypothesis A: There is also a group of "Watchers," (think "Highlander" rather than "Buffy") who study these demigods. This explains not only how the Professor is fluent in actual speech of languages that supposedly no Westerner has heard in centuries, and that very few have even seen crude approximsations of in print, but also... pretty damned much everything else about him.

Sub-hypothesis B: There is a duo of souls, not an individual. Which is yin and which is yang, I have no idea, but there is a tie between the two. And wherever they are born, they will be drawn toward what they must learn, and when the region needs them, they will be drawn back to it. And between them, they only get less than 300 IQ points. This particular incarnation of the Professor got lucky, since this particular incarnation of Gilligan only uses about 80 points. Hell, come to think of it, there may have been some kind of supernatural malfunction that only this particular generation split the line into two diections. Perhaps the Kupa Kai elder gods decided to try for a backup plan after they saw their reborn god shove the 123rd crayon up his nose.

The castaways are not really trying to leave the island.
Because they fled Hawaii ahead of an incoming Soviet nuclear attack. This is why the Professor has usable equipment and the Howells and Ginger packed so much clothing. Other than the primitives on the next island, most of the stuff they encounter is either their own imaginations running wild or supernatural/superscience After the End weirdness typical of 1960s-1970s disaster scifi.

The entire show is the delusion of one of the castaways who is adrift in the SS Minnow.
One castaway survived the storm that the SS Minnow encountered and was the only one left alive onboard after the winds died down. They are adrift w/o adequate food nor water (it was only a three hour tour after all) and is suffering from exposure,dehydration and as a result of hallucinates about the island and all of the "adventures" that happened there. Which explains why the escape plans never work,potential rescuers are all sociopaths and relatively simple ideas like patching the hull and/or building a raft are ignored. Take your pick as to which one it is.

Gilligan and the Skipper are not actually lovers
They're actually only pretending to be gay in order to gain Ginger and Mary-Anne's trust in a wacky plan.

The castaways are just ordinary people that happen to have become stranded.
They're not representatives of the Seven Deadly Sins or of the seven stages of drug intoxication or anything of the sort.

Gilligan is Maynard G. Krebs' middle name.
After being kicked out of the house by his father sick of his son's aimlessness, Maynard made his way to Hawaii, where he found a job as a first mate on a tiny boat. He went solely by his middle name.

Gilligan doesn't want to leave the island.
Being stuck on an island with the other characters is the best social situation Gilligan has ever been in, and he doesn't want it to end. This is why he sabotages all attempts to leave the island, but works so hard to keep the others safe from threats on the island. And as pointed out above, the show isn't called "The Island That A Bunch Of People Were Stuck On".