Try Sno-Bol. The toilet bowl cleaner works wonders and is inexpensive.
Recommended to me by a local gel coat - fiberglass man.

"Roland" wrote in message
om...
Just pull a small sea ray out and didnt belive the green grunge (for
the want of a better word) on the bottom and power washed much as i
could. Still a light green ammount left and looks like a weekend of
scrubbing. I tried checking archive for ideas but grunge didnt come
up. Please would anyone give some pointers on a solution to use to
help remove this; and to clear product i could apply to help stop this
next year. TIA, best regards...