Yes, rum has a very low freezing point. I kept reading many posts as to the drama involved in getting your freshwater in the springtime to taste decent after using environmentally friendly anti-freeze the previous fall. Somewhere I had read about someone using booze! Wonderful idea.

Start emptying the freshwater tank. Have someone watch the water level so that it is starting to go empty at water outflow . Then dump 2 L of cheap RUM into the tank just as the tank is going empty with residual water. Start fresh water pump and start monitoring by taste water coming out of the tap. If it takes like rum,
voilà, you have winterized your tank.

I would suggest you continue to pour your rum back into the original clear empty rum bottle. If you see floating particles, yuck!!! I happen to have a cappuccino machine on my boat. I poured the rum through the cappuccino filter attachment and dramatically reduced the number of floating chunkies. Yes, the metal funnel of the cappuccino machine had an internal very small diameter filter that helps cleanse out the line and pump mechanism.

Residual rum tasting of the tap didn't hurt the performance of the attending owner.