I've installed two HVAC units on my sailboats. The first was 9000 BTU Webasto FHC on my 30 foot S2. It is a decent. if somewhat-undersized unit. It's a bit loud. I had trouble with the v-berth getting more air than needed, but it was sure nice to nap in there under a blanket when it was 100 degrees outside! I also didn't like that the fan always ran. I'm now installing a rather pricey (for me) Ocean Breeze system (
https://www.oceanbreezeac.com/) into my B411. I went with them based on others' recommendations, the fact that they were the only folks I could find offering resistive heat on their bigger units (this is 18,500 BTU), and because the fan can be set to auto or always on. HOWEVER, I'm really mentioning it to you (and other interested parties) because the blower on this unit is extraordinarily loud. The boat's on the hard, so I've yet to give the unit a full test, but just running the fan makes me think I may well not be able to live with it. It also, as near as I can tell, has only one fan speed (it has a "fan speed" button on the thermostat, but it sure seems like the two speeds are "on" and "off"). I have this under the forward section of the starboard settee, and, with the cushion off, I was reading 78 DB using a phone app. I suspect there are much better solutions for you, particularly in the smaller sizes. If I feel any different about the unit once the boat's in the water, I'll let everyone know. Marine Air would probably be a good choice, based on what I've read in various forums.