Engine knock

My engine started knocking a couple of weeks ago so I took it in to see what is going on. Mechanic told me it was spark knock. So he replaced spark plugs and adjusted the timing. He told me he put the timing back to factory specs but it still knocked some so he retarded the timing until it stopped knocking. I took the boat out today and it was not knocking but I think since he retarded the timing so much the engine has started to lose power. I want to adjust the timing back to factory specs so I gain the lost power back, but e engine will begin knocking again. since retarding the timing solved the knocking problem it is a pre-detonation problem. What else could be causing this problem. It is not bad gas I burn a tank of gas every week and I always put 89 octane in it. Could the carburetor need to be cleaned and adjusted? Is it the distributor? I don't want to take it back to the mechanic he has pissed me off so I was looking for some advice on where to look next. The engine is an Indmar assualt 310 it is carbureted. Any advice would be welcomed.