I was thinkin' : We have tons of hostile mobs on land, why not water? How about a crocodile? Here's how it works. Crocodiles can only spawn in water areas 8 X 8 or bigger (They are not this big, they just need open space). They swim arround in whatever pond/lake/river they have made their home, until a player or passive mob walks too close to the water's edge, then, the croc will leap out of the water (Sort of) and attack. If you are more than 3 bricks away from a Crocodile's pond, it won't attack, but will watch you. If you are more than 12 bricks away, it will ignore you. When killed it drops Crocodile hide (Can be crafted into armour thats better than Leather, but not as good as any other armours), and has 12 hearts (Croc skin is tough, no?).
Thats my idea, hope you like it.