about reheating of the water

Our client was mentioning wanting the ability to fill his tub in the evening and soak, and then be able to use the same warm water in the morning. Is there a system that re-heats the water or maintains a higher temperature?
In Japan most of the hot water heaters are equipped with a re-circulation circuit and have a small pump built into the system.
But of course we cannot export a HW heater with the tub (you would not be able to do maintenance or find spare parts) so the only way is to build a system ad hoc.
This can be easily achieved by putting together some machines used for spas such as a pump, cartridge filter and post-heater (gas, electric or heat pump).
US is the land of private pools and spas so you have the best selection of equipment with makers such as certkin, jandy, pentair, waterways etc. If you prefer a low tech approach, you could cover the tub to avoid heat loss due to evaporation and in the morning you can add 10 liters of very hot water (60deg. centigrade) to raise the temperature.
Please let me know if I answered your question.