I use a Magic Eraser when needed. It all boils down to the water environment you're putting the boat in and what collects on the hull. I do know that if you do not clean the Cypress gum/tar off after you finish fishing in a lake with Cypress tress (or even a lot of duckweed), it will get to where it permanent stains the hull. A neighbor that I got interested in the Pro Angler bought 2 of them for himself and his wife to use. They have lakefront property and he was leaving the PAs on the dock uncovered. All the cypress droppings got on the tops of the PAs and now they are permanently stained.
I can't speak as much for the saltwater environment as I only fish there a few times a year, but the freshwater environment can vary a lot depending on where you live and the water environment. I'd be willing to bet you would never get any stains fishing Lake Amistad down on the Texas/Mexico border as that water is very clear.
The key seems to be cleaning the PA after each outing and not leaving it stored dirty.