One thought (obvious to most, probably but it just occurred to me) is that if its to be used as a live well, the pump has to be submersible (unlike the Jabasco "water puppy" that I have in there now...). Having that pump hanging there really limits the amount of water that can be in the hold. Also, having the sea cock in the actual hold makes for a weird setup, too.
The factory deck hatch leaks water very badly, even with a gasket all around it. After a good hosing down of the deck, there will be several gallons of water in that box. with a water-tight deck hatch that was easy to open, it would probably make a great "damp but not wet" storage area for cleaning supplies, fenders, lines, etc. Mine has a couple of brackets installed that appear to have at one time supported two vertical "baffles" which divided the space into three compartments.
I have a feeling that forward of this box is a large void that would make for good storage. Aft of it, the entire under-deck space is taken up by fuel tank, I believe.