Sea Monsters - On July 21, 2010, Notch commented via Minecast that Sea Monsters are a possibility. Particularly, he noted the possibility of sharks, but has recently stated that he would not be releasing sharks as it would be too difficult for the players to deal with.
Also, further along in the page:
Whales have recently been confirmed as a sea mob, although their purpose hasn't been specified.
0
A quote from: http://www.minecraftwiki.net/wiki/Mobs
Also, further along in the page:
Guess it's just a matter of time :smile.gif: