750 on a '98 is about average. If the owner can document service and maintenance, I would not be concerned. Looking at the boat will give you a great insight as to how it was maintained. I've seen 20 year old boats that I would rather own than some 5 year old ones.
naych, hours don't matter as much as maintenance. I just bought an '05 with 620 hours but it was regularly maintained, actually it was even stored at the previous owners dealer when not in use. The engine is in terrific shape and still got a great deal because of the high hours. A properly maintained boat with high hours can be a great deal.
Not really a fair comparison of hours on a boat to a car. A boat typically turns twice the rpm of a car and has a lot more resistance. That crap of a boat has the worlds largest radiator is just that crap. You still have a thermostat and it still gets to temp.
You should be able to pile on the hours as long as maintenence is done properly. As people have said before if the boat appears to be well taken care of chances are they maintained it. If it looks like it has been abused well they prolly did not maintain it.
I have a book that I keep track of every thing I do to our boat. What parts have been replaced and why. The amount of hours on the boat at the time of repair or maintenece. Works good for me.