One thing to remember is that swimming pools provide incredibly challenging conditions from a building physics perspective. The air is saturated with moisture at a very high temperature, condensation will form on any cold surface, and the chlorine doesn't make things any easier. So especially for exterior walls or ceilings, the surface needs to be accessible for inspection and maintenance. If they cover a whole wall with foam to resemble rockwork, all sorts of fungus could begin developing behind it without anyone noticing. Structural elements, ventilation systems, pipe systems, filters, pumps, and electrical systems would also need to be accessible. And with all that heat and moisture around, and the aforementioned chlorinated environment, plants aren't easy to keep alive.
Plus, as mentioned, there are regulations. With every surface wet and slippery, the theming should not invite climbing, ideally not running either. They can't have many nooks and crannies in case evacuation becomes necessary. Lifeguards need to have a good overview of the pools and slides. There must be handrails for all stairs.
There are tons of little challenges of building an indoor water park, but given the restrictions I think this has turned out amazingly well.
