I am certainly no expert as some of these guys who have responded are, but my take on the leakdown situation is:

If you have the percentages, they should have told you if the leakage is through the top end (intake/exhaust) or the bottom end (rings). Last year, I was looking at a particular boat which had a 502 with around 380 hrs. I wanted a compression and leakdown test done before purchase. It was checked at a very reputable dealer in AZ., and I was told that the leakage was about the same as yours, all through the intake and exhaust valves. Mechanic said that it needed a valve job. Again, I am not an engine guru like some of the guys here and I was a little skeptical at first. The boat had been sitting up for a while and I really didn't know how long or hard it had been run before the leakdown test was done, but all the numbers indicated a problem with the top end of the engine. Anyway, long story short, they did the top end, I bought the boat and it runs like a new one now.

As stated, once you find out if it's the top end, bottom end or both, you can use that info to make an informed decision in deciding if you want to purchase the boat or not. If I really liked the boat and it needed a little TLC, it would not bother me to have to do a little maintanence. I would just use that as a bargaining chip to negotiate price. Good luck.

Did they warm the engines up before doing the leak down?