Probably no manual ever came with it that would have any kind of schematics on it. Your going to have to trace the water lines to the tank. Starting at the faucet or outside fill tube. Most of the time they are in the engine compartment ( straight inboards )set up to off set some sort of weight distribution if need be.
If you can't trace any of the water lines to the tank and you have opened every nook and cranny and still can't find it , sell the boat and join some boat club .
