We been a live aboard for 16 years in the Seattle Area. Rather hard to insulate a boat, but you can hold in the heat and reduce draft and cold surfaces. Most reasonable size boats have a alternate primary heating, propane/diesel, to supllment the AC heat and when the dock power is lost.
We found the best ways are:
Carpet the floors to hold out the cold from the bilge. The water temp is about 50 degree so the bilge usually does not freeze or need heat.
We close off/doors and usually not heat areas not used so the heat is kept confined in the living areas. My wife made small quilts that we hang over the stair ways, salon to pilot house and state rooms to salon to hold the heat down, since heat rises.
Install/place the heaters at the lowest level. 50% of our heat is the middles of the boat at the bottom of the stair ways from the staterooms to the salon as heat rises, and all heat sources are at floor level.
In the winter the big salon windows are covered/protect with plex a glass which cuts down the draft and hold in the heat. We also have heavy thermal curtains.
The exterior canvas and tarps keep the cold wind/rain off the boat, reduces drafts and hold in some of the heat. On the cold/clear/freezing days its quite warm.
The best investment for a live aboard is to have a good primary heating system and/or have the most the most dock power amp the dock/marina will allow.
Not relating to insulation and retaining heat.
Condensation/moisture can be a problem so cooking/shower/washing is keep to a minimum and have to air out the boat, so you don’t want to make the boat air tight, but reasonable tight so air flows though the boat. We keep several of the port not dogged down tight.
Make sure the boat is easy to get on/off and in/out with only one hand. The marina facilities. Restroom/Laundry/general store as you will use and depend on them. Be aware that most marinas shut the water off for weeks when it freezes, and the dock power can do out for days. So having large capacity water tanks can be important and learn to conserve water.
Also its nice to have a sanitation/black water pump our boat/cart/system that can service your slip. All the marinas we have lived in have sanitation pump out boat. We get pumps out weekly if the marine is not frozen over and the pump out station is not frozen. We have a 45 gallon holding tank and we use the marina restrooms during the day.
Most things on a boat are a limited resource. Electricity, water, sanitation, heat, space, refrigeration, etc so you have to learn to adjust your wasteful land/dirt habits. Be sure to talk to the other live a boards and the marina so you have some idea what to expect.