There should be a lever in the water opening one of the grates, that lead back to the area where you met Rethpian (the dwarf you probably gave the rat-pie to). It's close to the grate, under water, and I think it's not the grate where you found the key for the four tombs, but the other one (but i'm not 100% sure here).

Go back to the four tombs. Cross from the Eastern tomb, dunk underwater, and follow the outer wall (keeping the wall to your right). There should be a tunnel underwater with a lever along the left side and a gate at the end. If there is only a gate, keep following the wall until you find the correct tunnel (there are only two).