Thanks for the quick responses. I realize I made a mistake in saying it was the raw water system. It was actually the fresh water cooling system. Sorry about that. IN addition, I should say that I replaced the heat exchanger in the Caribbean 6 months ago due to leakage at the end covers. The Genset has 3000 hours. The technician who recommended the procedure is not a Genset specialist. He's more a general boat technician. I'll have to ask him what the additive is that he is recommending.