When you get knocked of a corner because you are too fast and you dive in the water in the minecart you will reach the bottom of the lake/river/sea very fast. When you reach it in survival mode you die, because you hit the ground too hard. Minecarts should slow down in water and I don't think you are supposed to die.
ThortheM 3.4.2014
Minecarts are made out of metal and sink very fast.