Some basic points that many miss in indoor pools:

1. You must design the pool so that all elements of the build are EITHER
==> kept warm to prevent condensation, OR
==> waterproof, corrosion resistant outdoor construction.

2. Use treated wood and approved fasteners throughout.

3. Use bathroom grade sheet rock.

4. Use outdoor wiring techniques. Use outdoor lamps. Do NOT use embedded light fixtures; it's almost impossible to avoid condensation & corrosion if you do.

5. Place as little wiring in the pool enclosure as possible. Do NOT place electrical breaker panels in the pool enclosure.

6. Oversize the filter, undersize (or right size) the pump; use a 2-speed motor; run on low 24/7, except when cleaning.