I'd cut the floor open and repair or replace the tanks, as having more fuel lower in the boat is best.
As most leaks occur in the bottom of the tank, capping the fill, vent, and fuel line will create something of an air bubble inside. Of course it could have a hole in the top, but a camera should help reveal more. Than again putting a camera inside the fuel tank thru the fuel sending unit hole should reveal what failed.
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