Also, the door shown in the water area, you leave it open.
The room is 2+ deep, you can jump down the mineshaft and kersploosh. The ladder will block water in its square, and the door will keep the water from flowing out (but you leave it open).
The ladder on the side is so you can climb it, then you have an opening to get to your main ladder (so you don't have to try swimming against the current to climb back up)