Open the deck plate, and look to the stern, do you see the bulkhead at the threshold to the lower berth? Is there anything else up to the bulkhead besides the bilge pump? The cabin floor cover comes out in 5 minutes , take the screws out and knife line the perimeter seal, with deck plate open, reach in and pull up on cover,bias pull against one corner, presto it's out. Never bed that cover in place, just caulk the line after it is screwed in. GE Silicone II window and door, color of your choice. I use clear.
If not, do you see the black hose coming down from the floor drain in the head floor? Should be laying near bilge pump.
My boat didn't have the shower option, but I do see the flat base pad by the bilge pump where the sump pump assembly goes. I recall it being to the stern of bilge pump.
I'm upgrading this winter to hot water and the sump assembly already bought fits there fine, and the black hose coming from the head floor drain is already there. Just have to add power feed, thru hull and hose to it.
I would not believe Grady dumps the grey shower water into the forward bilge for the bilge pump to take care of. Heard of some sailboats working this way. Bilge must always smell like shampoo, I guess.
Perhaps the previous owner added a shower but no sump pump assembly.
You have near zero access to the area under the head, except from the starboard gunnel side, will be lucky to get your hand past the hull to enter area. I didn't consider cutting out the foam for other access from the stern side if possible. but Grady would have made the sump pump accessible for service, there is a filter screen in it to attend to. The port side is blocked by a stringer, the forward side is blocked by the floor in V berth ahead of head. Rear is blocked by bulkhead and/or foam.