Clean4u,
For wax, I use 3m's heavy carnuba marine wax. I think megs makes a marine wax too.
To get the water stain out, use an acid cleaner with Hydroflueric Acid, same type of acid cleaner that is made for cleaning and restoring aluminum. Spray it on and rinse off after 5 minutes, that should take care of it nicely and with minimal effort. ONLY USE ACID CLEANERS ON NON-PAINTED SURFACES, fiberglass boat hulls can be painted but usually are not, if you don't know, don't use it. Also, keep away from polished brightwork, and glass. When using, if it gets on the trailer, rinse it off immediately. You can even use it over vinyl decals, shouldn't hurt it. Will not harm plastic.
You may have to do the whole boat with it though. It's amazine how an acid cleaner brightens up fiberglass. If you just do the water line area, you may notice after cleaning it that it looks much cleaner and brighter than the rest of the hull.
I've seen many a detailer compounding and polishing, when all they needed to do is clean it with an acid cleaner and then seal it with wax.
My whole response is valid for RV's and anything else that is fiberglass or nonpainted aluminum.