As the above posters stated, the majority of marine engines die from neglect and improper maintenance. Most pleasure boats will see less than 50 hours per year of engine time. An engine that is properly maintained and used conservatively will easily last 2000-3000 hours, which is 40 to 60 YEARS of use in a boat! Of course if you like to run WOT all the time, no different from a car running WOT all the time, expect engine parts to go BOOM sooner.
However, there are a couple of critical maintenance points on boats that are not present on cars. Lots of people try to save money by doing their own maintenance. Unfortunately they often try to also save money by not buying (or downloading) a factory manual.
Biggest killer is probably winterization. Can you imagine how many cracked blocks there would be if people had to use straight water in their car cooling system and drain it any time there was a threat of freezing?? That's the situation you have in a boat. If you don't know where all the drains are or know to clean the rust out of them so they CAN drain, you'll probably end up with a cracked block.
Second biggest killer is probably overheating. Waterpump on a car has a metal impellor that doesn't wear out, and if the pump goes bad it's obvious by the puddle under the car. Boat waterpump has a plastic impellor which has to pump sand, rocks, shrimp, or whatever else is in the water. It's lubricated by the water it's in, and people have a bad habit of running the engines OUT of the water and frying the impellor. The plastic has to flex every revolution of the pump, and it gets brittle and breaks up over time. Changing the impellor is a maintenance item not present on a car which is routinely ignored by many boaters until their engine starts smoking ......
Failure to routinely replace expensive exhaust manifolds and risers is probably the third biggest killer. Especially when used in salt water, the iron manifolds naturally tend to rust away internally. First warning many people have is when the engine sucks in water and hydrolocks, hopefully not throwing a rod through the block. Hours of use aren't really a good indicator of condition, pump salt water through it once and let it sit there a few years, don't be surprised if you need new manifolds.
Having salt water leak into an engine is nowhere near as nice as leaking some antifreeze into it. If you get salt water into your engine internals through a blown head gasket from overheating or a leaking manifold and don't realize it or do something about IMMIEDIATLY, then the engine is pretty much history.
Pretty easy to avoid all three of these conditions. Know how to properly winterize, change your impellor every couple of years, and be aware that if you're running iron manifolds you WILL be replacing them sooner or later. They also make rust inhibitors (Saltaway) that you can flush through your system to minimize corrosion. I went with a closed cooling system and ceramic coated aluminum manifolds on my boat to minimize some of the above problems. I also have a water pressure gauge which allows me to keep an eye on the condition of my impellor. I'm going for the 3000 hours on my motor!
