Is this a fresh engine?, to be honest, the only way to really read plugs on a gasoline engine is to put some real hard pulls on the engine and "click it", then by using an otoscope to look deep into the plug shell at the base of the porcelain insulator for the color ring. Or even better, dyno tune the engine with headers with O2 sensors in each header tube, the load put on an engine on the dyno is real close to how a jet boat will load the engine. So if a "tune" is ironed out, it usually can be left alone in the boat. On one of the pic's I see a few dark peppered specs, this may be an indicator of detonation, this may be carbon that was rattled loose from a piston top, but I dont know your compression ratio. A 750, mechanical secondary is enough for that eng CID, and those jets are pretty much what that model came with out of the box, along with 6.5 power valves. Is your ignition strong enough?. Getting a jet boat to pull out onto a plane is a pretty good load on everything. How quick does your ign. advance come it? I have initial set at 16 and total is in at 1,800rpm and 20 deg. of spread in the weights put my total at 36 deg. Has anything in your combo been changed recently?..there are some unknown variables for me to really let loose on some possible solutions.