When you are crouching and you get knocked into the water (eg by a monster or player) you can not get out because you can not stop crouching. The crouch button disappears when you are in the water. If you are crouched and get knocked in, you just sink and there is nothing you can do about it in survival. You have to die to get out and loose all of your items in the process. I hope you can fix this.
Thanks