The metal can be guaged with ultrasound quite accurately. Where is the boat? My friend who is one of the world's expert's in ultrasonic testing just moved to Texas and he has some very nice guages. I would be more worried about rusting at the bottom of the tanks. If water got on the top of the tank, it also got onto the platform where the tank sits. It is possiable that the top could be replaced, but this is not where leaking usually occurs. With a top, one could clean up the top, grind down to good metal and make an epoxy glass repair. I completely wraped a steel gas tank with epoxy and glass years ago--I am sure that with time there was little steel left, but the glass held well and I essentially had an epoxy/glass fuel tank (It was an open boat--and I would not consider this in an enclosed boat). Also a new top could be welded on--if you can get to the outside part of the top. Bob Austin