QL77CC Champion gapped at .035 is what I'm finding. The L77JC4 looks like its the plug for a lot of J-Rudes at .030.
NGK BR8HS gapped at .032 is also specified.
Not saying the guy at the marina is wrong but find it odd he can tell you what plug but not the gap...
I'm not a Merc guy but the L77 sounded odd to me.
Good luck - I'd do some more checkin' as the wrong plug can really create a problem.
This is a question that Bullet will smack down in nuttin' flat...