Found it. Remove the stains into the v birth, open up the door, remove the floor board and voilà. Thanks for the helpIf you don't have any luck getting the answer here, there's a Monk 36 Owners Association page on Facebook. Click on send a message to see more than the photos that don't seem to belong there.
Meanwhile...I suspect that the "shutoff valve" is the seacock on the intake water thru-hull. To find it, trace the intake line from the toilet to it.
--Peggie