It might seem like an obvious question at first but it regularly confuses new hot tub owners. Most hot tub control systems have a maximum of 40º C and freeze protection that kicks in when the hot tub senses the water going below 6.7º C, which initiates the pumps and blowers to prevent the water from freezing. These are simply the boundaries that your hot tub temperature can be within, leaving it down to you to decide what you actually prefer.
Some Like it hot!
The maximum temperature of a hot tub is 40º C for safety reasons and any hot tub that goes above this is not fit for use. Some hot tubers will like to bathe in 40º water and within an hour or so of use on a cold night, the temperature will drop to around 37.5º, so this is reasonable.
We have noticed throughout the years that women tend to like the temperature slightly hotter, usually around 38/39º C. When delivering a hot tub, we usually leave the customer with the temperature set to 38º C, this is more comfortable to get in than 40º C and allows for a slight temperature drop whilst the hot tub is in use.
Firstly it is important to consult you doctor if you unsure whether hot water could affect you. Pregnant women should avoid hot water as it can damage the fetus. Also people with heart conditions or high blood pressure should not use a hot tub at higher temperatures. If you are in doubt, contact your doctor.