Hard to guess without a "sniff", but
It may be MEK ( from long term leagage from curing fiberglas ), in which case I do not know how to fix it.
or mildew ( much more likely ). Mildew need not be visible to the unaided eye to produce odors. A boat is often humid, and enclosed, whick is a wonderful vacation spot for mildew.
In addition to over-the-counter mildew remedies, a through airing out ( outdoors ), as well as letting it sit in the sun for the day ( MUST be dry ). The solar UV plus air will help a great deal.
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In the future, you need to get and keep better air circulation withing the boat, esp when it is "locked up" while you are not there. Stagnant air withing the boat can cause many issues, and odor is only one of them.
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Dave posted while I was ( slowly ) typing

, but yes, waste line permeation can also introduce a "special" odor.