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.