Tony, you won't have solved the problem, only temporarily cleared it. Check also your cooling water filter as some of the filters fitted are of a very poor design which can allow mussel shells through to the pump and beyond. A recent posting, and one I submitted a year or two back, noted mussel shells being found at the entrance to the cooling water tube nest and blocking much of the flow of water. When your boat is on shore you need to look through the cooling water slots within the sail drive, and to remove whatever marine growth you see within that area, and do it every time your boat is ashore. I have used a high pressure water jet with some success, and the other possibility is a long thin bottle brush (eBay), but you do need to check the flow of cooling water fairly frequently when you are on the move and under power. The first indications I had of a problem was where I wasn't sure if it was smoke I could see coming out of the exhaust or if it was steam. It turned out to be steam!
Several of the boats onshore this last winter I noted they had wrapped their saildrives in plastic bags, I don't know what if any benefit this may provide, but on the down side it would maintain a moist atmosphere within which, I'm guessing, might be sufficient to support the life of these little critters, and possibly sufficient to allow them to procreate !!!
