I used to get the “boat” smell too. I replaced the dorade vent on the coachroof with a solar vent (set to extract), and I also leave a few of the hatches on vent to encourage air circulation. It has completely got rid of said smell, and also helps massively with condensation and I have no “black spot” on the ceiling in the spring any more. Also worth checking that the bilges are completely dry, and if you are in a marina, try putting a dehumidifier in the boat, with the drain going down the sink.
