stains

I purchased my 2004 2575 RFS used and the toons have been neglected and left in a slip too long.  They have a paint on the bottom of the pontoons that has degraded over time and allowed algae to cling on.  Where there is no paint, the aluminum is stainded heavily.  I've done a number of things...

I have heard some horror stories about furniture being stained from sun tan lotions/oils & sun screens . I am going to protect it with 303 . I have also heard that sunscreen with " Titanium Oxide and Zinc Oxide " will not stain Any thoughts,comments or horror stories ?