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.