I'm up here in VT, not on the boat. I'm well aware of what cold weather
can do to waterlines.
I really don't feel you need to worry about your water
lines freezing as long as they're inside the boat. Even outside the boat, I'm not sure.
If you're going away for some time you might consider it. Or the boat is on the hard
If the boat is in the water even though the air temp is 20 degrees, I doubt if the water temperature is ever below 50. That water heat sink will keep your boat well over 40 below decks. most I'd do is shut off the pumps and open the faucets. The head
system should take care of itself.
A boat on the hard
is a little different. Inside a boat wind
chill isn't a problem. Just go with the actual temperature. My experience is that water will get slushy at around 32 or higher depending on other conditions. However, you need to be in the low twenties or high teens for a while to get the water to become the kind of ice that will really exert pressure on the lines. If you've shut off the pumps and opened the faucets what little ice expansion there is will be directed to the faucets. If you have a water pump disconnect it and let it drain the water is trapped in small spaces in the pump so expansion is limited.
Remember, even on the hard, although the air temperature will drop to the 20's for the evening the ground will still be above freezing and the boat will be warm during the day. I doubt if you'll see much below 30 degrees at night.
Hope that helps.