Here's some interesting info on anti-fouling paint from the
West Advisor (West Marine).

NB
http://www.westmarine.com/webapp/wcs...-fouling-Paint
BTW: You will have to remove ALL the old paint anyhow, before you apply ANY kind of new paint. Once you remove the old paint, you WILL have to apply "Some Kind" of new paint because the bare gel coat will be very unsightly after having had salt water anti-fouling paint on it for years. The old paint fills in all the stress cracks in the gel coat and will look like varicose veins after the old paint is removed. The lines can not be removed..so you paint it again. I've been there.