Dennis, the key part to get out is the alum on the left side of that panel, not from the formed fiberglass that stays as one piece.
"The AS manual says, "remove the panel adjacent to the shower in order to access the water lines for disassembly and removal of the water heater"....
This is piece of luan to the left that also comes out.
If I recall it is also screwed into the floor, it will come out by bringing the bottom edge towards you.
When your done the fibgass #15 and the alum are one part, the luan the other.
I assume you removed the toilet
If not I hope you know a good chiropractor