Re: how many hours is too many?
Boat engines will easily last thousands of hours IF THEY ARE PROPERLY MAINTAINED! I don't think I've ever seen a "worn out" boat motor, they usually die from a catastrophic maintenance-related failure.
A boat motor lives in a much more corrosive environment than a car or airplane motor. To keep it alive requires some preventive maintenance.
Most die from overheating. If you don't like to spend the money to change impellors, water pumps, and hoses regularly BEFORE they fail, don't be surprised when your motor welds itself into a solid lump. When the boat stops running and there's steam coming out of the engine compartment, you waited too long.
Lots of northern boats (and some not so far north) die because of cracked blocks. If you leave it full of water and it freezes, you get to buy a new motor. Just as a note, there is no such thing as "freeze plugs". Those plugs in the water jacket on the block are CASTING PLUGS. They're used to get the sand from the casting mold out of the block. If one happens to pop out and save your block, you got lucky!
Lots more die from water in the cylinders. Put off a tune-up until the motor starts "dieseling" when you turn it off and there's a good chance it's going to try to run backwards and suck water into the cylinders. Put off changing the exhaust manifolds and risers because it's too expensive, and an internal slow leak can ruin a motor within a matter of hours.
I just pulled a 1986 5.7L apart that had decent maintenance - nothing special. It has 630 hours on it. It had 150 PSI compression in 7 cylinders, 110 in the other (new spec is 150). The bad cylinder had a cracked piston, looked like something had been dropped through the carb into it and had bounced around on top of the piston. The spark arrester was missing the hold down nut and nobody knew where it went. Imagine that. All cylinders looked like new, even the one with the bad piston. No wear ridges at all. No reason that engine won't run for thousands more hours with one new piston.
Took apart a 1981 2.5L with over 1200 hours on it about 6 months ago because it was locked up. #4 cylinder was ruined because of a leaking exhaust riser. Piston rings were rusted solid to the cylinder wall. Had to break the piston apart to get it out. The other 3 cylinders looked great. All 4 cylinders had a very slight wear ridge, about what you would see on a car at 50,000 - 60,000 miles. The guy tried honing #4 and putting it back together with a used piston. The front 3 were 130-140 PSI compression (new spec is 140), the bad one was 90. He's been using it like that on a private lake all summer, it actually runs good except that it bogs on take-off if you try to accelerate too fast.
What all the above rambling is trying to say is that if the motor has good compression in all cylinders and runs good (no knocking, etc), then it's probably in good shape. If he can't tell you if the manifold and riser were ever replaced, it's probably about time to change them.
