Here's what you need...
http://www.jamestowndistributors.com...Bottom+Cleaner
But be REEL careful when using it...has about 3 different acids in it and will cause immediate respiratory distress if inhaled...some BAD stuff, but it will get the caterpillar poop off your boat...read the directions closely before you use...Academy Sports also has it...I got mine thru a marine dealer..