SSP and SMP Bug:
When you dive in at least 3 deep (or more) water and touch the ground you cannot go up again.
You get stuck in the ground and drown.
Happens in every gamemode and every hole filled with water. The size does not seem to matter.
Just in the sea it happens only sometimes.
I think it's duplicate of MC-1648 (and a lot other).
The new pre-release messed up the water.