I'm moving aboard this weekend, and the seawater intake for the head in the master stateroom is frozen shut. It's something I'll have to deal with, but in the meantime I can tee into the sink drain. Hopefully, it's below the waterline.

There are two more heads on the boat, and they're hooked into one intake line with a closed seacock. I followed that line until it disappeared to someplace not easily accessible in the forward stateroom. I was figuring on having to use the bucket-and-chuck-it head. Thanks Peggie!