start with basics,vacume guage look for leaks,check timing,check valve adjustments,sounds as they may be to tight.compression test all cycls.

x2, and check fuel pressure at the carb.

if you can remove the valve covers easily i would check there first,if you do decide to change the csms go with the 270h,i just installed one snd the owner loves it,i would not go any bigger than the 270 with stock exhaust.

Is this boat Alpha or Bravo? If Alpha, check the shift interrupt switch - sometimes if out of adjustment or hitting ground these can cause intermittent idle and running issues.

Was this a hydraulic flat tappet cam? Before you pull it down, pull the oil filter off and check it for metal. If you rounded off one or more lobes, it will surely show up here. Much easier than pulling the top end apart for no reason.

If the cam is wiped, consider going roller the next time. The GM dog-bone roller lifter arrangement is cheap and nearly bulletproof, and GM has some off-the-shelf factory roller cams out there that would work well in a marine engine and aren't that expensive.