Would not leave a heater on for extended period of time, winter boat fires happen. Fit a valve that will isolate raw water
and take a hose fitting...after sail when motor
is hot put the end
of hose in bucket of anti freeze and run motor
till the anti freeze
starts coming out and stop motor...then leave with peace of mind...motor will not freeze during prolonged cold spells and you don't have to worry about a heater which may fail or worse.
Installing valve is quick and easy.
Dockhead: seems that I remember some palm trees in the
southwest of England! Northern Delaware climate a little inland
on the C and D canal is somewhat colder. possible as much as
20-25 degrees F in a extreme winter
I refer to the the agricultural zone charts