This is one of those where I just say "it's magic". Otherwise, you have to go into all kinds of stuff about why the caster isn't fried as badly as the target. If you want it to be useless in water, make it useless in water. If you want it to work the same, go for it. Any answer is legitimate, it's your world.
Otherwise, my next question as a player will be, how does my invisibility spell work in water? I mean, water and air have different indices of refraction. My Leap spell (drag?) Sound spells (sound travels differently in water)? And how did the caster manage any verbal components, if they can't talk underwater?
I'm as willing to speculate as anyone else, but there's really no "right" answer when magic.