Depending on the type of boating ya plan on doing the 3.0 will work but it won't be anywhere near a speed demon with only 140hp. The 3.7 would have been a much better choice with an aluminum head but the rest of the parts are becoming more rare and very expensive when found. Mercury really stuck it to everyone when they scrapped the 3.7 instead of finding a solution to there problems. I let them know at 2 international boat shows back in the nineties when my exhaust manifold went bad and there was no replacement made. Second time I was nearly arrested but got my point across to higher ups in the Corporation.
My option these days is to install a 4.3 in the '85 but don't think it will fit in your application. If money wasn't an issue,I would drop in a diesel if I where you. Best of luck with your quest.