With regards to Larry's response, the water heater drains easily, it is the tempering valve that did not survive.

When the boat wintered in Maine, we blew out the lines. The tempering valve though has some spaces closed off by the valve, unless you could warm it above the set point while you blow through it. It didn't work when I recommissioned. I can't say for sure that it was damaged by freezing, but that is my suspicion. Nothing was broken or leaking, but it no longer regulated temperature.

In the Chesapeake, I disassembled the valve, which is quite easy: The knob is held on by a concentric screw. The underside of the know has features the limit its angular movement, so you have to remove the knob to unscrew the stem. You want to keep track of the number of turns you unscrew it, so you can put it back the same number of turns. The stem will screw out, underneath is the wax pellet (enclosed in brass) sensor and a spring. I left them out over the winter, but you could probably just blow it all out and reassemble immediately. It did not freeze as hard in Virginia as Maine, but the tempering valve worked fine again on recommission.

After Maine I bought a new valve, rather than disconnect the plumbing and swap out the whole valve, I simply disassembled it and swapped the wax sensor in from the new one. That fixed it. So freezing jammed or damaged the sensor?