tile cleaner for bathtub works... and it is a lot less noxious than the hull cleaner.
the joys of having a white hull in tanic water!
TSP is a good bathtub cleaner. Works like a charm, never tried it on the hull though? Might be a good idea as the stains from water on the hull are the same as in a tub. Calciam, grime etc.. I do know that once you get it off if the area of Q is nutorious for discoloration from water try buffing it back to shine with the Dimond Cut 2.0 then apply several layers of wax over that area and always wipe that spot dry after each time out and that should prolong the discoloration from comming back for a very long time. Any hint of it comming back nip it in the but right away.
The only area i have ever seen some color change that needed attention is behind the fenders aft. That because it the hardest part to get with wax while on the trailer.