Re: What kind of wax should I use on an old fiberglass boat?
+1 for sanding first! I just did a few spots on my 76 Thompson as a test. I started with a good wash and wax stripper to remove any dirt and wax, then I sanded with a red fine grit Scotchbrite pad, then some good 3M rubbing compound applied with a small buffer followed by some 3M automotive polish. Really brought the paint back! Of course, now I have the rest of the boat to do.

As far as using marine-specific wax/polish, I can't comment one way or the other, as I've never used it before. I'm sure it has something to do with being wet or submerged all the time. But yeah, if you want a really nice finish, you're definitely gonna have to sand the finish lightly to remove 100% of the oxidation.