right after this I found Simon in a grate past a waterfall I hadn't shot at. You smack the wooden pallet and go through a crack in the wall. Fart to scare away the mice. Then fight off the Mama Rat. --- 3DS FC: 4957-3929-7962
On the same level as the sewer fall you have to go behind because you cannot turn it off keep going to the right even when you reach what looks like a stop, get against the wall and walk behind the tubes.
There you will find a ladder going down and a gated tunnel. The latter going down is the latter to Cornwallis.