Note: This is the same Mystic_Eyes from minecraftfoum.net. My original topic can be found here. I'm interested in hearing what you guys think of the idea, so I'm mirroring it here.

Like their IRL equivalents, Nurse Sharks would be non-hostile unless threatened. I'm sure that many of you are scratching your heads wondering why sharks of all things should be implemented as a neutral mob. I based that premise off some things Notch said, where he didn't want to implement hostile sharks (see the Notch's streaming (July 14, 2010) section). I'm well aware that Notch isn't an active member of the team, but there's a good reason for no hostile aquatic mobs due to the massive advantage even a small group would have over the player.

Sharks will only spawn in wide spaces of water (in rivers and oceans) and only on dirt or sand. Therefore, you're not going to have any sharks invade your cobblestone swimming pool or have to tiptoe around them, trying to pick up crops that fell in the water. They would spawn only in unlight (not restricted to night) areas and would travel a lot more during the night.

When struck, a shark will chase and attack the player for moderate damage. They are immune to conventional methods for taking critical damage (the usual jump + hit) but recieve more damage if you strike their nose. They also will resist being pulled by a fishing rod, but if moved out of the water they will flee and slowly suffocate. If killed, sharks drop leather. Or possibly some kind of shark meat (which I'd imagine would be stronger than pork/steak due to killing a mob that fought back.)

Note that I'd imagine the fish being a smaller and having the thickness of a held tool. But there's only so much you can do in Techne before distorting the actual model itself.
One interesting aspect is that sharks will rarely spawn with one or two fish floating along side them. If they spawn during the rain, the likelyhood doubles. Players can use shears or a fishing rod to pluck the fish right off the shark, but if they do, the shark will become angry and turn hostile. If killed while having fish float around them, the shark will drop the respective amount of fish.

Also if you want to use this idea for a mod or something just post a reply/pm me first.