If it runs well at top speed, and that means it can get to 4500-5500 RPM in gear in the water, then you have proved that the fuel pump and fuel supply is OK. If it starts well and idles well but stalls when the throttle is advanced the there is likely a clog in one or more carb(s) Do not run the engine until you find the problem!
Remove the carbs and disassemble them. Clean well. Check the float for free movement and check the inlet needle for free movement also. Put a cheap in line plastic filter between the fuel pump outlet and the carbs. The fuel pump can shed rubber off the diaphragm and this will clog the inlet needles. If it does this to only one carb that cylinder will run lean and destroy the piston--ounce of prevention