Just a note about boat hours. One comment in this thread states a concern that if the engines have 300 hours then so does the rest of the boat. Too true. However if a boat only has 50 hours it may have many more hours on the rest of the boat. It depends on the style of the boater.
The hours on my boat engines tend to reflect total hours on the boat, or perhaps the boat has about 20% more wear on it in hours used than the engines. Why. Because we tend to only go out for a few hours at a time. We live close to the lake. It's easy for us to make a decision at noon to go out boating, be at the lake and on the water in less than an hour. Then we'll tube, knee board, etc. for two hours, then head back home. On those days there is very little time on the boat with the engines turned off. On a rare occasion we might pull into a cove on the lake, anchor and sit for an hour or two. So our boat hours are just a little more than our engine hours.
I know other people who take their boat out, anchor, and use the boat as a swim platform, eat lunch, or just keep the boat at the shore for long periods while they are on and off the boat all the time. Many people love the jet boats for their wonderful swim platforms and sit there with the table installed and beer in hand. They might jump on the boat to take kids for tube rides for an hour or so but spend most of their time at the shore or rafting with other boats. Those people might put two to five times more hours on the boat than the engines. In this case a boat with 50 hours of engine wear might have 250 hours of boat wear.
I encourage those looking for a boat to examine the whole boat, as I know you are doing, but don't worry too much about the engine hours. 300 hours on a 6 to 8 year old boat just isn't a lot of engine hours if the boat was well cared for. There are numerous examples of jet ski rental companies with 1,000 to 2,000 hours on their rental Yamaha jet skis. If it was a car 300 hours x an average 40mph is only 12,000 miles. A Honda or Toyota is just getting broken in at 12K miles. Even 1,000 hours in a car is only about 40K miles.
Much of our concern about engine hours comes from 2-stroke days where engines might only last 200-300 hours between engine builds. They were fraught with oil / fuel mix problems, carburetor issues, fouled plugs, lower crank case pressure, and the like. None of those problems exist in these super reliable Yamaha engines.
I would have no problem buying a Yamaha jet boat with 500 hours as long as the rest of the boat looked great.
As captainhook said he has his eye on a boat with low engine hours but it's been sitting for a couple of years. That is a much bigger concern than engine hours. Were the engines just left for a couple of years or were they run from time to time on a hose? Were they fogged before letting the boat sit? Was the cover U/V protection enough to keep the sun from doing damage? Did animals or insects get into the boat and do damage?