Rocket Power/WMG

The whole thing is the 'Super Squid' videogame created by Sam
If you see the episode "Sim Sammy", where Sam mades a parody videogame and then decides to do a more accurate one, it make sense that the whole series is the version 2.0 of the Super Squid videogame that Sam created. It would explain the Totally Radical nature of the show, the horrible physics implied and why Sam always tags along with the group, even when his mother is a very overprotective one.

Sam Doullard had Asthma or a severe alergic reaction
Oh, come on... there is a lot of hints to this one. In the first episode one of the first lines spoken by Sam's mother is to remember her kid not to forget his inhalator. Also being moved from Kansas to Santa Monica in California implies that something forced this moving. In adittion to this, the sea breeze is effective in some cases of asthma. However he manages to recover from it. This explains why at the begining he is too bad and then improves into a competent athlete: He is cured from the original illness.
 * This is confirmed in the episode "Big Air" which centers around Sam's asthma, after it lands him in the hospital the rest of the gang decide to "take it easy" with him, though Sam eventually convinces them that having asthma dosen't make him any less fun.

Sam will enlist in the Navy when he grows up.
The rest of the group will join other branches of the US military. Thus, Sam will still get called "Squid".