I see a lot of these modular swim spas going indoors in garages, sunrooms and basements.
If you plan for humidity, it shouldn't be a problem. Install a flooring that can handle splashes and high humidity (i.e. tile, hardwood or indoor/outdoor carpet). Install a venting system similar to what you would install in a shower. And most importantly, be sure to install a security cover on rollers and a track. Close the cover when not using the swim spa. If you do all three of the above, humidity will not be a problem.
All of the manufacturers of modular swim spa discuss the need for floor drains on indoor installations. They do this for a good reason - to minimize their liability if something goes wrong. I've seen a number of floods on indoor installations and they fall into two groups: 1) silly mistakes and 2) slow leaks. It is very very unusual for a swim spa to have a catastrophic failure. When there is large, fast flooding, it is always because the homeowner had done something silly (i.e. fallen asleep while refilling, or left a valve open while refilling). However, slow leaks are somewhat common. These are caused by bad pvc fittings, leaky through-wall fittings, cracks in the shell, or seam delaminations or punctures in vinyl liners. To sum it up, if you can easily install a floor drain, I would do it. If not, try to avoid silly mistakes, and install a sump pump and / or a flood alarm. You should be ok.
Not sure what you mean about electrical requirements. All of the swim spa manufacturer's have electrical specs in their installation instructions. In addition to these, I do have a few thoughts. Many municipalities will not allow you to install a swim spa in a spot with electrical mains running overhead. Also, I would avoid any location underneath steel beams or metal piping (if the swimmer touches these, they could act as a ground, completing an electrical circuit running through the swimmer).
Hope this helps. Good Luck.
