There is a difference between the temperature of the water, and the temperature of the surface of the water bottle. Even moreso, most water bottles have a bladed "2nd surface" (don't know how this is called) that further separates the actual surface from your skin, and allows air to transfer a large portion of the heat towards your skin at a lower rate.
If you want even more insulation, you could wrap the hot water bottle in cottonsheet or some other natural fabric. This has the added benefit of preventing some of the toxins from the water bottle rubber/plastic from entering your body through your skin and your lungs.
Keep in mind that some hot water bottles have an upper limit on what temperature of water they will be able to handle without a high degree of likelihood of bursting open. Also, keep in mind that the hotter you make the water, the more likely you are to get serious burns if the hot water bottle does burst or puncture and spill. Most hot water bottles recommend an upper limit on temperature and also on volume. If you fill them all the way with hot water, they're more likely to burst if you have them under you rather than on top, and the weight of your body applies pressure.
50-55C should be fine in most cases. You can go higher, but again, the higher you go, the riskier it becomes. In most cases, never use boiling hot water.
The main advantages of going higher is you can use more insulation, and that the water bottle will stay hot for longer. (you may need to remove some of the insulation as it cools down). However, if the material the water bottle is made of is toxic and releases toxins, chances are, the hotter the water is, the more toxins will be released.
