If enclosing (building) an indoor pool, careful consideration needs to be given to the amount of moisture that evaporates from the water in the pool and causes high moisture conditions, which are very tough on any wood in the room or in the walls. Moisture inside, cold outside, condensation on and in the walls will cause mold, bacteria, and wood decay. Paint peeling off outside siding and wet insulation, etc. are some more headaches.
I tried to plan one in my new home of 35 years ago, and could not swing it mostly because of cost, but also because I couldn't figure a way to get around the high moisture problems. Probably the best thing that could have happened to that idea. However, the basement floor in the 16' x 40' rec room is still ready to be carved out for its installation. Hmmmm? Haven't thought about that for awhile, until you mentioned it.