I know I am coming to this post late but THANK YOU for posting your pictures! Has helped me with the same issue on my boat (84 Hunter 31)
Today I removed the lid of the freshwater tank, I found multiple places where it was leaking. A previous owner had sealed the lid under the mattress so water found another route, sideways between the layers of Marine Plywood exiting at the wall on the bedside opposite the Fuel Tank. Sealant removed, Tank and Lid Bleached, and Reseal tomorrow.
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