You cannot get a natural tan indoors. Tanning requires ultraviolet (UV) radiation from the sun (specifically UVB for melanin production, UVA for immediate darkening). Indoor lighting—even bright waterpark lights—emits negligible UV.
Indoor waterparks with retractable roofs (some locations). Only *open-roof* sections allow real sun exposure—but still filtered by glass/structure. Not reliable for tanning.
Want a tan? Go outside. Use SPF. Want fun? Kalahari delivers—sans tan.