Um, carbon fiber is used quite a bit in racing sailboats. Carbon fiber masts and spinnaker poles are very common these days. There are growing numbers of racing boats with carbon fiber hulls. I think this is a well understood issue, and carbon is quite safe in salt water. Most boat owners use a bottom paint, whose purpose is primarily to retard growth on the bottom. More and more boats are going naked on the bottom though, in lieu of a toxic bottom paint. Me, I still use VC-17 with a bag of pure copper dust mixed in.