Zedz is right. If the engine has high hours and has not seen regular oil changes, then I would look at the cam/lifters.
There is a slight, and I mean slight adjustment available on the roker arm nut. There are several theories on how to set them. All work.
I use a stock valve cover that I have cut the center out of, and install it on the cylinder bank that I am working on. This keeps some of the oil from getting everywhere during the adjustment process.
Fire up the enigne, and SLOWLY back off the rocker arm locknut until you hear the rocker start to clatter, then run it down far enough to quiet it down completely, then back off just slightly, keeping the noise in check. If you can't get it to quiet down, or must really crank the locknut down to do so, either the cam has worn a lobe down, or the lifter is bleeding off hydraulic (oil) pressure, or both.
Run a static compression test, if the numbers are close to even all around, and
if the engine has reasonable oil pressure, which, in my mind is 10 PSI per thousand RPM,and good idle oil pressure when hot (15+) then I would do a cam/lifter swap, and take a good look at the cam bearings while I was at it.
A cam and lifter swap is doable for the average backyard mechanic. If he buys the boat and plans to keep it for a few years, I'd do the timing chain/gears, cam and lifters. Over here that stuff would set you back about $400.00 for parts through one of the large mail order house operations like Summit.
Good luck with it...