In your EntityCrab class, put "this.setSize(widthOfCrab, heightOfCrab);" into the constructor. You need to give your entity a hitbox otherwise it has no collision with anything...
public EntityCrab(World par1World) {
super(par1World);
this.setSize(1f, .5f);
this.experienceValue = 4;
this.tasks.addTask(0, new EntityAISwimming(this));
}