For really tough boat hull cleaning you will need to break out the acid. Muratic acid is the best for this. You'll want to have a 50/50 mix of muratic acid of 6-8% concentrated (12% is the highest concentration you can legally buy over the counter), water and a good strong sprayer. You can purchase muratic acid at Wal*Mart, K-Mart, Pool Supply Stores, Home Depot, Lowe