assuming the computer used to run these engines is similar to a car. the computer will detect the knocking/pinging and retard the timing to compensate for it. as long as it continues to detect, it'll run in a retard state and you will have some sacrifice in power. I don't think anyone will ever notice a boat running full tune and one running with the timing advance in a retard state, but as mla suggested, this will have some longterm effects on the overall wear of the rings/pistons/valves and seals. most engines today have quite a bit of "safe" room in the tune tho..

went to 2 tuners over the years to get the old mitsu evo's tuned. it was fun to watch the pc hooked up to it real-time detect and retard the ignition when picking up on knocking/pinging. one was on the interstates out by the salt lake city airport and done while driving. tuned the last one to the brink at a shop in portland on a dyno. original baseline dyno was 330hp/310tq. when I left, it was at 363hp/362tq with no other parts swaps, just an ecu tune. shows how conservative the engine tunes can be. sure mike has even bigger gains from diesels and tuners.