Is the VRO still hooked up or do you premix? If that cylinder is running with low compression it can make the motor run like crap out on the water while running under a load, BUT it may run fine at home on the muffs.
I had a Merc 150 V6 that had one cylinder w/low compression. It ran great on the muffs. I didn't run it under a load for fear of causing the piston to break apart or throw a rod through the block.
I had HOPED it was just a stuck ring and decarbing would free it up. The compression never came up so I tore it down. The ring had welded itself to the piston and gouged/scored the cylinder wall. This was from the previous owner not doing any preventative maintenance, had a clogged jet in one of the carbs which caused it to overheat.
If I were you, like I had previously posted, take the head off and check that cylinder before running the motor anymore. If the low compression is from a damaged ring, piston, etc., the damage WILL get worse by running that motor.
If you don't notice any damage you can pour some seafoam or "ring free" on top of the cylinder, let it soak for a day or two, and see if it may just be a stuck ring. BUT if it WAS a stuck ring I would think that your recent run on the water would have broken it loose.
So go ahead and check compression again. If it still low then pull the head. If the compression reading is higher than before, then it may just be a stuck ring.