I use Zing hull cleaner! The product has been around since 1948 or so. It is the best cleaner I have ever used. Squirt or brush it on and discoloration is gone, no rubbing. Down side is that it is acid based (BUT IT WILL NOT HARM THE FIBERGLASS) it will remove the wax. I have used this product several times and can tell you it works. I can only find it in the mid-West, I have not found a source on the East Coast yet.
It is great for the discoloration but can anyone recommend a cleaner that will remove the tougher stuff that we get from the bay? And how do you remove those dam barnacles.