I use a product called Super Clean for nearly all my cleaning on the boat. It''s made by the Castrol Oil folks, is biodegradeable, and safe for fiberglass. It does a great job of removing the stains that bird poop can leave behind, and also removing those black stains on the topsides. You can find it in the automotive section at Wal-Mart in spray bottles with gallon sized refills.
I don''t allow Soft Scrub on the boat anymore. It''s still too abrasive to use on gelcoat for me.