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Also, with respect to the seacock, another possibility is that the boat was not properly winterized and this seacock was damaged by freeze expansion of water. Not knowing where you are located and whether your climate makes this a possibility, makes it difficult to say how likely this possibility is. But you mentioned that you inherited the boat and boats coming from estates often have a degree of (understandable) neglect if their previous owners were very elderly or not well for a period of time.

And, the washdown/feed pump that is mounted immediately adjacent to the seacock could also have been damaged by freeze if not properly winterized. So this should be inspected as a source of a leak too.

So I mention these as additional items to check. If the seacock is freeze damaged, closing it might not help the situation. The boat would need to be removed from the water to be properly repaired.