I know how much trouble that crap can be. I am a commercial painter and sometimes we have to spray the poop with a stain blocker before putting the latex over it so it does not bleed through. You might need to try some sort of over-priced marine gel-coat cleaner. You could also try a more natural approach like maybe vinegar or lemon juice (that is just a total guess). Here is a link to a thread on boat detailing using a few of the cleaners
http://www.bloodydecks.com/forums/bo...t-looking.html