Spas

For interior spaces, most clients choose a pre-made "spa" tub.  The prime advantage is a reduced thermal
mass and usually superior insulation.  As such the water tends to maintain a constant temperature better.  
There is a very large variety of quality tubs - in many sizes, shapes, colors and with various jetting / bubbling
features.
cost disadvantage, a custom built spa will often blend
into a landscape much more seamlessly.  The process
is effectively identical to that of a swimming pool.  
Excavation is the first step, followed by re-bar, then
shotcrete, water-edge tile, then a form of plaster
coping.   In contrast to interior spas, particular
attention will need to be paid to filtering.  There are
three popular methodologies: Diatomaceous Earth
Filters, Sand Filters and Cartridge Filters, each with
their own sets of pros and cons.