My Yanmar 6LPA is in a lift-kept boat and gets a freshwater rinse after every outing. Even so, after about 150 engine hours over 2.5 years, the lower three zincs, which are always wet, require semi-annual replacement, while the upper two (fuel cooler and intercooler) are always dry when checked and remain pristine looking. The excellent mechanic I occasionally allow on the boat tells me he thinks the raw water drains from the upper parts of the system through the Tides dripless shaft seal cooling line from engine to shaft seal.