how to keep bilge from freezing?

How about a fish tank heater? Like this one? https://www.amazon.com/dp/B003C5TPF6 For 20 bucks you get the heat where it needs to go. The little heater has a thermostat so only the amount of heat needed is applied. have the same issue wiyh my boat and am considering doing this. For the 2 gallons of water that accumulate in my bilge before the pump kicks in, I calculated 100w would be enough. There are online calculators to determine how much power is needed to heat a certain amount of water. Keep in mind that salt water still freezes - but at 21F. I am not sure if this heater would qualify as 'ignition protected', but on a diesel boat it should be fine... I think.

 

If you're going to throw something in maybe propylene glycol. It's non toxic, non corrosive, doesn't produce explosive gases and inhibits microorganism growth.

 

I believe your easiest solution ( pun intended! ) would be to add salt to the bilge.

 

I don't understand how you can get rain water in the bilge if the temperature is so low. There may be snow on deck, but no way it is raining.