hows the rest of the boat look?  does it look taken care of?  hows the oil look, etc?  400 hours could also mean 400 hours at WOT and its worn out.  You can get a good feeling just talking to the owner about the boat and the overall condition.  Even if they're not tech savvy they should know the name of the place they take it to for oil changes and repairs and if they are tech savvy and did it themselves they probably wont shut up about what they did and how much they've spent.  If anything request to do a quick compression test to make sure the motor is in good shape.  If you're low on a cyl it could be big bucks.

The hours by themselves are not a deal breaker.  Like has been suggested earlier, it is how the hours were accumulated and how well the engine was maintained .  You can do a compression check pretty easily.  If you take the boat out for test, does the oil pressure maintain a steady level?  What is the oil pressure at start up and then after running awhile?  After running long enough to get oil temp up does the oil pressure drop significanlty when the engine idles?  All are indications of the engine condition.  There is a lot of information on this site.  Suggest you do a few searches.  If you are not real comfortable with engines in general, do you have a buddy that can help you out?  Good Luck with the boat.

I bought mine with 500 hrs on it. Had it out ten times so far and I can't imagine it running any better for what it is. It's a Marine Power 390 HP 454. I haven't done a compression check or oil analysis yet, but I'm not scared of it.

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