Could use some insight to the mysteries of boat maintenance. I have two pumps that are a mystery. They are connected electrically and plumbing but I can't figure out what they are (or were) for. This on is behind the engine water muffler in the engine room. This one is under the sink in the head. Is this a zinc on the top of the heat exchanger? Either it's just a plug or the zinc is completely gone on this one. If it is a zinc, what size and where can I buy a replacement?