I installed an Xtreme heater in my bilge last year and they work great! The operation is inexpensive too! I am in GA where the temps do not reach critical very often so I am not sure just how these would be in a colder climate. My boat is in water year round. I do seal the vents when the temp is predicted to be below freezing and remove the seals if I want to go out.
I use a Xtreme heater, 450w model on my Bayliner 2455
I just drain the manifolds, plug in the heater and go home