Thanks for your reply's.
Yeah first time out this season and under lots of load it started to miss fire, load reduced ran fine.
Replaced rotor, cap and leads, went out and miss fired about 45mins into our ride, not loaded. Was pretty bad, at higher rpms it seemed better but aborted the day as a precaution.
So have new plugs and a coil with module, fingers crossed otherwise it's off to service guys and throwing cash away most likely.
Will take one of the old plugs out and see what gap they may have been then set the same for the new ones.
We get out a fair bit 6-12 times a season each trip we're out for 4-5hrs only and the engine shut off if we are not going somewhere.
I look at the few new boats on the lake and turn green with envy but at 250K plus nowdays I'll pass and enjoy what I have.
Thanks again, will advise once I find the fix, hopefully.
Cheers BruceR, Melb AUS.