http://www.youtube.com/watch?v=rl-flAi31Yo
The waters are no longer safe...
A picture for those who are too lazy to watch the video:

Current Info:
Sharks come in about five different sizes.
Sharks normally drop shark teeth, which currently have no use.
Large sharks have a 1 in 10 chance of dropping a shark egg. Throwing this egg into the water and then waiting a bit will cause the shark to hatch, growing from newborn to adult over a period of 10 minutes. This shark, once full grown, will not despawn- allowing you to make a moat full of sharks to protect you from creepers.
They attack anything that falls in the water but squids and other sharks.
Future Info:
Sharks' attack range will be greatly improved, to 32 blocks instead of the current 5-8. (this is a bug.)
Sharks will be able to break boats.
There will be at least two more species of shark.