I use a Xtreme heater, 450w model on my Bayliner 2455. Seems to work good. I don’t keep my boat in the water and I use a small electric cabin heater set to the lowest setting in the cabin. This keeps the cabin in the 50’s. I have all the cabinets opened and seat cushions up so the heat can get everywhere.

Xtreme Heater Top View
Xtreme Heater Top View