My custom entity, no matter what always sinks to the bottom of any pool of water and just sits there awaiting its death.

 

Here is all the AI code:

 

this.getNavigator().setAvoidsWater(true);
	this.getNavigator().setCanSwim(true);
	this.tasks.addTask(1, new EntityAISwimming(this));
	this.tasks.addTask(2, this.aiSit);
	this.tasks.addTask(3, new EntityAIAttackOnCollide(this, 1.0D, false));
	this.tasks.addTask(4, new EntityAIFollowOwner(this, 1.0D, 6.0F, 3.0F));
	this.tasks.addTask(5, new EntityAIWander(this, 1.0D));
	this.tasks.addTask(6, new EntityAIWatchClosest(this, EntityPlayer.class, 8.0F));
	this.tasks.addTask(6, new EntityAILookIdle(this));

	this.targetTasks.addTask(1, new EntityAIHurtByTarget(this, true));
	this.targetTasks.addTask(2, new EntityAIOwnerHurtByTarget(this));
	this.targetTasks.addTask(3, new EntityAIOwnerHurtTarget(this));

	this.setTamed(false);

Hmmm, not sure.  You might want to try setAvoidsWater(false) instead.  Logically swimming and avoiding water are different things, but perhaps there is some conflict in pathfinding if it tries to avoid water while it is in it.

No change, perhaps it is just something to do with forge or maybe the hitbox, I am not sure.