My wife, Jacqueline, has been there, too. She's small enough to crawl entirely in. It used unusable space and provided ballast. To fill that awkward space, we designed a very complex-shaped tank. After 15 years and 40,000 miles, our tanks' aluminum was Swiss cheese and the closed-cell foam was a sponge.
Saltwater reaches the tank from two sources.
Foremost is the shaft packing gland. Here it is necessary to build a waterproof dam connected to a drain hose going over the tank and into the sump. Second is the sump itself. Fiberglas covers the tank about two-thirds of the way up, but a full bilge can reach the tank. The seal must be extended to the top of the tank.