Hi my name isXXXXX will assist you.
It looks like you have done everything except clean out the fuel tank. Since the introduction of ethanol to the fuel boats with old fuel tanks and systems have issues with the ethanol dissolving all the old residue in the fuel tank creating a sludge that restricts fuel flow to the engine. When the fuel flow is restricted the engine will not produce full power and will stall at full throttle operation. Here is an article on the affects of ethanol.
There is a very easy test to check the fuel tank run the engine on another fuel source. If the engine runs better then the boats fuel system is the problem. Clean the fuel tank and the screen on the end of the pick up tube.
If the engine runs the same then we have to do a compression test and check that all cylinders are firing.
Post back with findings and questions.
Get a fuel pump that is ignition protected, remove the fuel sending unit. With the pump and some fuel hose and some copper tubing you can use the tubing as a wand an suck out all the old residue. This will be time consuming if the tank is real bad. I have done many fuel tanks this way and works real well.
Post back with questions.