Towards the top of Map 2, there's a spot where the rubble has spilled out into the water, almost all the way across. It looks like we might be able to cross there, once we make sure that that stones aren't going to sink under us. With the amount of decay in that area, the wall might have crumbled enough that there's a gap we can squeeze through. If not, at least we can get to the dry land alongside the wall, and follow that till we find an entrance or a good place to climb.