Caustics....
I prefer caustics containing sodium silicate .... dissolves grease, dissolves mildew and fungus, removes air polution 'specks', removes 'old dead' wax, etc. Spray on, let soak, re-spray then scrub, then rinse. It will temprarily turn white gelcoat to a 'yellowish color' but is only temprorary and will rapidly fade back to its natural color. Then follow=up with a 'wash' of oxalic acid (hardware store/ paint store - 'wood bleach') to remove any stains from tannin and iron/rust staining. ........... also will remove the 'grey' from bare teak. Commercially available 'brands' would include "Tuff-eNuff" (about $9.00/ spray bottle) ... sold in many marine chandleries. Tuff-eNuff is a VERY good detergent for goats .... also good for removing interior mildew, etc. as it *dissolves"/liquifies the mildew cells. Once the interior is cleaned, simply spray on and let dry to prevent subsequent mildew growth. The oxalic acid is 'generic'.