There are fuel tanks , and there are boxed of fuel.
For a gas boat a box of fuel is fine as the consumption is usually high and the engine doesnt mind a bit of water in the fuel.mIGHT STUMBLE A BIT BUT NOT STOP.
Diesels get unhappy with water in the fuel , SO THE FUEL MUST BE DE WATERED.
This is usually done with filters , but the filters fail Big Time if you get a batch of poor fuel, or a deck fill leaks.
Another solution is a day tank, where only the cleanest fuel is used to fill it.
The S&S tank is not off the shelf , but in a remote area at least one of the tanks should have a real sump you can drain the water from.
Remember polly tank installation is a 2 step procedure . The tank must be filled with fuel for a day or two to expand before you foam it in place.
Perhaps just a custom day tank?
Yacht Design - Type Plans - Sparkman & Stephens