I am going to post up an old pic of my 4.3 when I had it apart about 10 years ago. The pistons show carbon build up (the Q-Jet was running too rich then for sure) and on the center piston you can see the lack of pock marks, this engine at that point probably had at least 1500 hrs on it, had one bad overheat and the blown head gaskets a few years later, but it doesn't seem to show detonation damage, at least from what I could see. The liquid in the cyls is fogging oil to keep the cyls from rusting. Block still needed some cleaning in that pic!
Maybe the water in your manifolds was a blessing in disguise, if not looking for things you'd probably never had noticed that.
I would: consider getting the injectors for your TBI system cleaned, make sure fuel pressure is what it should be
check timing
see what your WOT max RPM is, if too low you have to start looking for causes, sometimes it's as simple as the boat ran OK with the wrong prop on it and no one ever thought to check, on mine it had a 14.5x19 which was too much pitch, it needed a 15x17 given the little 4.3 and the weight of the boat (a lot for a 20')
Another cause which could be easy to miss, is the ratio of the outdrive, each engine has it own specific outdrive gear ratio like a ring and pinion on a tow vehicle. If the outdrive was replaced, someone could have used one with too tall of a gear ratio, so that can be a cause too.
use at least mid range fuel, in a marine engine you really can't hear pinging or detonation but it will eventually burn valves and can shorten engine life.
