No insulation in my 2003 356. The Marine Air works well in 40 - 44 degree water, below that I don't have any experience. I cruise in late fall and early spring. I have a generator so when I am not at the dock I run it and the HVAC continously. Once the boats surfaces inside get warmed up and you keep the HVAC on, it is good to the high 20's outside and maybe lower. It was 27 this morning when I got up and the boat was comfortable. During the night, it got to the low 20's.
Water temp was 44 last night and the boat was 45 inside when we got there at 9 PM and we stayed at the dock, plugged in. By 10, it was at 72, but the interior was still cool due to the cold surfaces. This morning, the chilled surfaces were warm and by late morning, was very comfortable in sunny 38 degree temps. I have a full enclosure and that keeps the cockpit warm when the sun is out and also helps warm the cockpit fiberglass. It took a few hours for the rear bedroom to get to ambient temperature and we had some extra blankets. We were very comfortable during the night and by this morning, the bedding was fully warm. If we had been out cruising, we would have had the benefit of the heat from the generator. Mine is located in the port lazarette (5KW Northern Lights) and that helps heat the foot of the bedroom.
Don't know how cold waters and temps you plan on cruising in, but I strongly recommend a generator, enclosure, autopilot and handheld AP control. On really cold cloudy days, I stand watch in the companionway and use the hand held autopilot control to steer. I keep the slider tight against my chest, wear a heavy coat with a hood and the heat rising knocks off the chill. I am also behind the dodger and I stuff small towels in the holes where my furling lines come through the dodger. You are also over the engine and get the benefit of the heat off it too. The interior stays pleny warm in this scenerio. Those few things make all the difference in the world for comfort. I have sailed in 25 knots at 40 degrees on a sunny day and been comfortable. Returning last spring up the TennTom, left Midway Marina one morning at 27 degrees, but we had been on the boat for about 3 weeks by then and the interior was very comfortable and after a couple of hours and sun, the cockpit got up to about 50 in the enclosure.
