Re: Hours on a boat??
Assuming this is an outboard, I wouldn't blink at 800 hours in 6 years. better to run them than to let them sit, and what they do to the boat when it's not running is more important than when it is. Chances are, if someone ran it a lot they also made sure it was running well.
There is no support for the comment "rode hard and put up wet" suggesting hard use and neglect. Ignore that.
The yamaha manual suggests maintenance at 1 year/100 hour intervals, suggesting that it expects 100 hours a year.
Here are two ways the high hours might affect the motor, assuming ordinary use (like not towing houseboats):
if it was trolled a lot, you can get carbon build-up. That is easily remedied. Budget in a carb job.
If it is underpropped, the hours may be effectively 10 times that. Call the guy and get the prop size (diameter x pitch) and look it up to see if it matches. If it's the original prop and he bought new you are probably OK.
check and replace the LU oil and replace the water pump (impeller) before you run it, unless he has reliable info that he did.
High hours on an outboard, in sandy sea water at that, is nothing around here.
if it's not an outboard, ignore me. Go find a boat that has one, though.