The plywood salon floor extends all the way out to the hull and is about 10" below the fiberglass deck as you have found. As Flywright suggests it's a good idea to locate the deck plate hole and the location of the access hole in the plywood salon floor extension below, before you hole saw out the larger holes.
Depending on the deck fitting you choose, you may need to cut a second hand hole, near the hose hole in the plywood floor, to clamp down the deck fitting and attach the hose/clamp from inside the engine room. If the fitting is held in place with screws, then you also need a backing plate. Whatever fitting you use it has to be totally watertight. A water leak from the deck would lead to dry rot in the salon floor.
Careful . . .I Have a Generator and I'm not afraid to use it !