Not necessary to take the tank off the boat.. but do close the manual valve on the tank(s). The vapor pressure goes down with temperature (only 45 psi at 0F, versus 150 psi at 90F) so the tank is safer in the cold than in the summer.
Yes, propane does freeze, but not at temperatures you'd likely encounter... -306F..
Edit to correct my conversion math in propane pressure at 90F
