2001 Volvo penta 7.4GI

I have a 2001 21 foot Larson 270 with a volvo penta 7.4L GI engine and it will not come to plane. When in neutral, the engine revs up fine up to 4000rpm+ and purrs like a kitten. However when out on the water and accelerating it feels like it's misfiring and will not come to plane because the engine won't run above 2000-2200 rpm. We have tried changing the spark plugs (with proper gap), plug wires, cap and rotor and ignition coil and nothing changed. The fuel pump was replaced about 3 years ago with an aftermarket one and changed again after these issues for another aftermarket pump and no change. This boat has also always been bone dry in the bilge under the engine. This year it seems it is filling up. I vacuumed out all of the water and it came right back. What am I missing? Do I have a raw water problem where it is leaking into the bilge and causing the engine to not run properly under load? I can't send the boat to the shop and be without it for a season, we have several jobs to do out on island properties coming up. Also fuel was also treated when winterized same as every year. What do you think?

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