Using a light bulb to keep your lines from freezing is a sure way to lose your boat. One large sailboat at our docks was lost
for just that reason. Not only are there power surges and outages but, guess what? Light bulbs burn out!
If your bilge water and pipes inside the boat are freezing, then you don't have enough heat on the boat. You either have to properly winterize the boat, or you have to keep heat on it.
BTW, I had 1/2" copper piping also for my fresh water system. Also, marina water should NOT be left connected to your boat when you're away. Another good way to lose the boat.
I lived aboard for 17 years, and have had my present boat in winter conditions for 10 additional years in an area where it gets very cold. That's 27 winters. We sometimes get near zero F temps for days at a time. Often we get ice; once we got 22 inches of ice which lingered for almost two months.
If you have reliable electricity you could try safe electric heating
. What worked well for me for many years were those oil-filled heaters which look like radiators, set to the LOW or MED power, not the 1500 watt setting. They must be secured against the boat's movement.
However, these are vulnerable to power outages, too.
Best plan is to winterize the boat properly so you don't have to worry about it.