Do not stoop to using a hot waterbottle! This means carrying extra weight in the form of the bottle, fuel water etc!
Use the small of your partners back instead - my wife uses this technique on a regular basis!
A hot rock, well wrapped in a cotton or wool cloth makes a field expedient bed warmer -if you have no partner/single sleepingbags/small hammock, but be sure to use non explosive rocks, wrap them well (to avoid burning yourself or your sleeping bag as well as keeping your sleeping gear clean) and you can get away with leaving the rock behind insread of carting a bottle around!
I once avoided hypothermia by constantly using a NATO waterbottle as a hot waterbottle - refilling it with hot water as soon as it cooled - when I had a bit of a bivvi-ing epic in the Beacons one year....
Nowt wrong with a hottie bottle - lots wrong with dying!