I lived aboard in Norfolk for a complete winter. FWIW, had two smallish electric heaters for a 35' boat and never felt comfortably warm in the boat all winter. They just heat the air, and not all that well, but don't dry out the air. Condensation
was terrible and high humidity never gave me a feeling of being warm.
If you can, install an externally vented heater like a Dickenson Newport
. These types of heaters will heat the air and draw moist air out of the boat lowering the humidity.