Water Is Blue

Dean: Everyone knows the sea is blue. Ask anyone.
Ridcully: That's right. However, while everyone knows the sea is blue, what everyone usually sees is a sea that's grey, or dark green. Not this colour. This is virulent!

—Terry Pratchett, Ian Stewart and Jack Cohen, The Science of Discworld

In Real Life, you probably would have your misgivings about drinking a glass of some blue substance that presumably is clean water, but in drawn media this is just the way it looks (unless it's unsafe for consumption). It's not just large bodies of water with proper light diffusion to provide that colour; swimming pools will always look like this too, someone might as well have spilled dye into the tap water, and even tears are not exempt. It's just easier to animate it this way.

This Trope is near universal in Western Animation. Comic Books and Anime are more varied in this respect, and may even feature both solid blue and colourless, transparent water in the same art style. Generally, only older or cartoon-styled Video Games feature blue water.

It should be noted that water does in fact have an extremely faint inherent blue tint in Real Life; the color just isn't noticeable unless you're looking through a great deal of it, and even that has more to do with the diffusion of light as mentioned earlier (same reason the sky—sorry, make that the sky is blue). Check The Other Wiki for more info.

Also, some household chemicals are specifically dyed blue to discourage people from drinking them. Blue is most likely used because very few drinks are coloured this way.

Aversions include tingeing it white instead, or going the whole way and animating it as a distortion of the light. If water is deliberately not blue, however, chances are it's Grimy Water.