For the night mode coming in 1.0.5
you could make the players fatigue run out much faster because the water is cold, which will eventualy lead to the player drowning quickly at night
In the real world, water often takes a long while to cool down at night. I could explain the science behind it, but to keep my comment short, I'd still like to say I agree with your idea. The water would be cold enough to start hypothermia about two to four hours after the sun goes down (translated to game time, approximately 1 and 1/2 to 3 minutes after the sun goes down).
Hypothermia itself should also make the player a bit hungrier for a while even after they get out. Perhaps a 3 to 4 minute hunger debuff? The torches and other fire-utilizing items would help the debuff go away faster.