It varies - kind of like city MPG vs. Hwy MPG.
Was the previous owner a cruiser? Like a Cadillac owner that travels in style, stops to smell the roses, and doesn't beat the hell out of the boat?
Hot rodder? Lots of zero to WOT changes? Wave jumping puts huge loads on the driveline and engine as it free spins in the air and then takes a shock at re-entry into the water.
Speed demon? Redline RPMs and WOT. Jet is in the water more, but the engine is spinning fast and hot for extended periods.
Water flooding - hey even the pros do it just like the novices. This is like an old sports injury if it is recovered correctly. It doesn't get better, probably gets worse, but you can still play. But if you "play-through" and don't purge the water right away or correctly, then bearing life is shortened, rings score up the cylinders, etc....more extreme cases blown seals, screw up rotary valve.
That said, my 96 HX has way more than 500-600 hours on it and I think I did everything above (and then some). Catching my carbs clogging and going lean may have bought me a couple hundred more hours before the top end is up for a rebuild. But I also religiously fogged, winterized, and lubed the boat.
If the previous owner REALLY did maintain the boat in the off-season as well as took care of it during the running season, you can get some great deals on 200 or 300 hour boats.