Fliers can fall in water if they're made heavy while over a bridge... I think. I lost the battle before the next turn so I don't know if they start drowning, but they definitely displayed the "splash" effect.
They don't drown, I just finished that battle and had that occurrence. The splash occurs because a Bridge is really a Water space with an object above it; and the gravity spell uses the impact effect of whatever terrain it's on.
Oh, so it's just a visual quirk? Never mind, then.