Hot water bottles are meant to be filled with boiling water and put in a cold bed; they're utterly outdated, amazingly cozy, and impossible to find in the US, which to my mom, is an utter travesty—so I bought her a cute vintagey iteration on a trip to Avoca in Ireland's Powerscourt (a famous estate outside Dublin with lovely gardens for wandering). The present went over quite well, and given NYC's current 30 degrees, I wish I had bought one for myself. Other reasons to use "hotties:" some are luxe, others are quite frightening, and all are at least a little bit eco.
We bought our hot water bottles in Switzerland and cute washable covers in Italy. And we use them every winter. Much better than an electric blanket.