Although engine hours may seem like a logical source to determine engine life and condition, it is not necessarily accurate. The National Fisherman had a great article years ago that identified engine life was directly proportional to total fuel burned during the life of the engine. The article compared diesel, four stroke, and two stroke engines. Obviously, the diesel would last longest based on fuel burned, then the four stroke, and the lowest life expectancy was for the two strokes. Technology and the quality of the engines have improved and I'm not so sure that a four stroke engine will last longer than a two stroke, but I am convinced that you do have to measure the total fuel burned.
So the original example of an F225 engine, one with 650 hours and one with 1600 hours: If the one with 650 hours ran hard at an average of 15 gph for the life of the engine, the total fuel burned would be 9,750 gallons. For the engine with 1,600 hours, if constantly cruising around the bay for an average of 6 gallons per hour for the life of the engine, the total fuel burned would only be 9,600 gallons burned. So, potentially, the two engines could be in the same condition.
I'd go beyond the engine hours and interview the owner. I'd want to know things like:
Was he the original owner? Was the engine broken in correctly? Was the engine professionally/properly maintained with records? Has there been a recent compression test? Was the engine flushed after use? Was Ring Free used regularly? Etc. I'd also look at the condition of the boat. That can tell a lot about how the engine was cared for. And finally, what is the difference in price?
Good luck with the right choice.