I have an I/O and had a leak from the rear seal of my heat exchanger. It looks like you have an O/B, so that's probably not it.
I'd check that through-hull. My wash-down fitting didn't seem to be leaking, but when I went to re-plumb the pump, it moved a little and when I pulled it out, the transom wood was wet and thinking seriously about starting to rot. I'm glad I checked it.
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 Scott, Portland, OR '85 V-20 I/O, Merc 170
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