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.
This works as intended. There is no reason for mobs not to go for a swim.
On the other hand, there are some modifications planned with the random movement code, so this behaviour will be slightly tweaked in the near future.