Surely hot water bottles are not made of rubber any more, but of some synthetic such as vinyl, which cannot perish.
I would prefer a "stone pig" any day of the week. Wrapped in a towel they are not too hot to handle, and if you take them out of your bed before getting into it and wrap tomorrow's underclothes round it the clothes are warm in the morning, or at least not chilled and the water in the bottle is still warm enough to wash your face in.
The electric blankets you put under your lower sheet tend to be lumpy to lie on, are all made of synthetics, which are a problem for anyone like me who is allergic to most man-made fibres and cannot be washed. Three very good reasons for not wanting one.
Can I recommend a nice warm cat? Or better still: two nice warm cats?