Enjoyment of hot water baths has a considerable history, as the above links will show. Hot tub spas have become popular in luxury gardens, far removed from sustainable design principles though they be. Cedar- and redwood-clad hot tubs are available for outdoor installation. Whirlpool effects are provided as an aid to relaxation. They come in 'personal' and 'party' sizes. Spa chemicals are used to keep the water safe. A dip in a hot spa tends to be invigorating if of short duration but enervating if prolonged.
The plumbing arrangements for a hot tup spa are similar to those for an indoor hot bath: there needs to be a hot supply, a cold supply and drainage. Electrical fittings for whirlpool pumps and jets require particular care for outdoor use (where the risk of physical damage to the wiring is greater).
Whirpool spa in garden
A whirling pool
Hot tub spa