I've only tested passive mobs, but prior to pathfinding changes in 15w40a mobs would avoid falling in water while wandering. If they fell in, they would seek land again.

In 15w45a, mobs will wander off of a platform surrounded by water, and not seek land again.

Can be reproduced by spawning mobs on a platform surrounded by water. Linked webm shows difference in behavior.

http://gfycat.com/TartOrneryHoatzin