The suction point for the tank is not quite at the bottom - Maia and I had to bail out the few gallons left after the pump sucked dry.
Then we find this mess (at both ends of the tank). These end bulkheads keeps the water in the tank - and these were very spongy. I'm surprised they weren't leaking. The solution is to tear away all the old rotten or spongy wood and apply a plywood doubler patch over the squishy area. Then coat liberally with several coats of epoxy. It has to be multiple coats to make sure no pinhole leaks occur.
Out comes my favorite tools for this endeavour - a 1" chisel and a hammer. It allows you to (carefully) peel off the tabbing from the wood baffles and the hull without damaging the underlying laminate. It is also a lot less messy than an angle grinder. If I was working on this tank with nobody aboard the angle grinder would have come out, I would have tented off the rest of the boat with plastic in the work area and had at it. But we are living aboard and the noise, inevitable glass dust escaping, and general unpleasantness of grinding glass means the slower chisel is the way to go. I got good enough by the end that I was peeling off strips of mat tabbing a foot long.
The hull internal laminate is in good shape by the way - a few osmotic type bubbles but nothing to worry about really.
You can see the white patches of mat where the baffles were located. Each baffle took about 2 hours to totally clean off. New baffles are sitting on the foredeck with the first couple coats of epoxy on them. Di is coming home tomorrow. Better keep at it...