If the engine is using a smaller bore carburetor with a venturi, you may be able to run it just fine with the fuel tank installed down low in the boat hull. I don't think you'll be doing loops or vertical climbs too much. But who knows, my old Dumas airboat used to get airborne if i wasn't careful.
But if the engine you have has a huge carb on it, you may have to go with a fuel pump to help get fuel up to the carb.
Now you could maybe use a bladder fuel tank. This is sort of like using a balloon and you force the fuel into the bladder expanding it. the bladder then forces the fuel into the carb. A number of racing and other competitions use bladders to good effect.

Another thought is to use crankcase pressure to pressurize the fuel tank. This was quite popular for racing, etc too. One way is to use a special pressure fitting off the backplate with a tapered rocket nozzle shape to the hole, of which the hole is smaller too. The pressure pulses from the crankcase are allowed out but the pressure doesn't get to bleed off quickly because of the hole or nozzle shape. Some engines had a place on the right side of the crankcase directly below the carb where you would drill and tap a hole to use timed crankcase pressure, in this case the pressure fitting doesn't need a special nozzle shape to it.
The Cox teeDee o49's have a small plastic fitting on engine right side just below the intake venturi that you could drill out for timed pressure purposes.


Here is a pic of a FOX .74 engine with timed pressure fitting location cast into the crankcase just for that purpose. You can see it below the carb just in front of the large part of the crankcase. Anyway more modern engines usually have enough crankcase material in that area to permit one to just drill and tap a hole in that area like for a Cox tee Dee engine. To drill the hole, you need to rotate the crankshaft so the intake cutout is open to where the drill will come out on the inside. You drill a small hole. Then carefully drill a slightly larger hole part way through., this you thread for the small pressure fitting, using a bottoming tap.
Timed pressure means you can hand flip the engine to start. But using a pressure backplate may require using a electric starter.