Davis Instruments makes a FSR ( Fiberglass Stain Remover ) that is amazing. The secret is Oxalic acid and it will not harm gel coats. It is in gel form and must be seen to believe. It will remove all stains completely with out any scrubing or elbow grease. Just wipe it on, wait a few minutes and stains are gone.

Works on everything from tanic acid stains from those nasty rivers, to diesel and oil stains from trailering. Tip Top Teak used to make a similar product for cleaning teak. It had the same specs as the FSR. If you can find it.
This in conjunction with the 3M wax Jake mentioned will have incredible results.

Good luck!
AJ
Nacra 6.0 Express