Water pathfinding problems:

• Mobs try to reach its targets in impossible ways in certain conditions when water is involved, even if there is a possible route to get to the player.*
  Example situation:
  A player stands on a platform which is surrounded by water. The platform is one block above the water and it has a staircase down to the water, so you can get up. A zombie spawns on an island near the platform and sees the player. The zombie swims directly towards the player and ignores the stairs. The zombie won't be able to reach the player.
  Note for example: This happens sometimes, depending on how near you are to the staircase.
  You can see this in "3" of attachments.

*To reproduce:

1. Build a platform one block above surrounding water. Build something that can take you up to the platform.
2. Spawn a mob in the water.
3. Attract the mob while standing on the platform. You'll see that the mob ignores the path that takes you up to the platform. The mob tries to get up to the platform but never succeed.

The attachments show another way of reproducing. Additionally, when replacing the water with lava in the example "1" of the attachments, the mob's behavior changes.

FIXED:

• A mob rather approaches its target by swimming, than to take the faster route.

• Mobs that you attract refuses to swim to reach you when you're standing on a platform surrounded by water. The mob never reaches you then(See "2" in attachments). An exception is when there is just a one block gap with water between the platform you stand on and the mob.