This one's tricky. Open up the map in Paint and select the pencil tool. Put the cursor at around 630, 190. There's a switch there. Hit it, then find a platform that tries to move upwards and crush you against some spikes. Ride it for a second, then jump off to the ledge on the right. Climb back to the surface and (drum roll, please...) there's a platform floating above the water that'll take you to the shortcut.
