There are two great dehumidifiers, both made by Eva-dry. They're peltier cooler powered(fewer moving part, cheaper but higher power draw).
I use the EDV-1100, the smallest available in my Alberg 30(liveaboard with 2 people/continual construction site and some leaks.) Paid 45$ on amazon, and it's been running 24/7 ever since.
I have considered upgrading to the edv 2200 if I can find the space for it, the water extraction rate is 3-4x better, for 2x the footprint and power draw however at this point I'm not sure if I need it, my RH seems to sit around 50%.
My edv100 draws 2.5 amps.
Positioning makes a big difference, I've tried all over.
-warm air, the extraction rate drops by half in cold air.
-space. I had it in a shelf with only about 2" of space over top for a week, I emptied it only once and it wasn't close to full, normally I empty it more regularly and full.
The only modification I've made so far was to remove the fancy grate on top where it exhausts
, it moves a lot more air without the grill
, and seems to extract a lot more water without it too. On the one hand I was concerned that the air moving faster would mean that it wouldn't spend long enough in contact with the cooling element to dry it, but the increased air movement was a bonus so I figure it evened out. Non-scientifically, I think it fills up less with the grate in, so it was a win in all regards except for being a bit more of a pain to clean some day I guess.
I have considered swapping the fan out for a quieter computer fan, and or looking online to find a very energy efficient peltier cooler, and using a high quality computer heat sink(more surface area) to make a better version of my own.
If I make my own, it will duct the air from cabin top to floor, the warm air at the top has the most moisture, and recirculating warm air to the floor is a very efficient way to make the boat stay warmer longer(you lose heat upwards, so keeping the warmth distributed in the air space helps, and increasing the temperature at the floor level makes the biggest difference in comfort). I would have the warm side of the peltier exposed with a small fan on it as well, this way 100% of the electricity used will be heating the cabin effectively.
I can't recommend one enough. The first month I had it on board I was emptying it all the time, now it fills up so much more slowly.
I had initially planned to have it in the galley and then drill a hole in the tank and let it drain to the sink but since it fills so slowly now I just dump the tank in the head occasionally. You can find ways to stay warm, but damp is much harder to beat.
You can buy the marine version(same specs including energy consumption as the Eva dry models) for close to 1000$, but I figure i can replace my 20x, and with no difference in energy consumption I see no benefit at all to going with the marine model.
The best change I made to my boat was buying those cheap brightly coloured interlocking foam tiles at Home Depot designed for kids play areas. We noticed a difference within an hour, no more cold feet on the floor, and the air around your legs is much nicer when warm.
They have two additional benefits.
1. they essentially seal the bilge off from the cabin, preventing the air/heat/moisture exchange that normally occurs.
2. The bright colours make it much easier to find the small parts when I drop them, and the foam prevents it from going down the holes into the bilge.