At an average of -8 make sure you have a low heater to keep through hulls and hoses from freezing and make sure everything gets drained really well.
I am in the water year round, but nowhere near as cold out here on the left coast. That said, we did have a couple of two week periods at -15 where the water did freeze in the marina but we just tried to break it up around the boats. Usually there was a enough latent heat in the hull to keep the water from freezing within an inch of it.
