Submerged swimming pool lights are designed to be built into the pool walls as they are being constructed - they are safe for swimmers because they are recessed into the pool wall. An underwater battery operated light would have to be fixed to the pool wall and would be a safety hazard for swimmers. All modern pool lights are 12 Volt and power is provided by a cable to a 240/12 volt transformer in the pool control panel. Lights can be installed retrospectively but the pool would have to be emptied down to the light level (about 500 mm below the water surface)

Yes planting and greenery can be a lovely addition in a pool house, there are some plants that thrive in that environment and others that dont do so well, but we have some very good gardeners that help with the selection and plant detail.

I did a search for garden pod buildings and amongst a lot of others, this one came up. It has prices on the website, so it will at least give you a sort of guideline of what a building like that would cost. https://www.smartgardenoffices.co.uk/?gclid=Cj0KCQjwvabPBRD5ARIsAIwFXBlhJvpztz64gP5jL7i6B3-nbbf01CdDfES4_-_M9d-6AZuuo_HXUzkaAk25EALw_wcB

Roof- talk to a joiner about this. I think they will suggest removing the roof, and replacing it with a timber structure covered with roofing felt. I suspect the pitch is too low to have skylights. And then they will probably lay floor joists over the pool and lay a new timber floor. The windows will also need replacing. I would be weighing up the cost of having them made to fit the aperture versus buying a standard size, timber framing the window wall and adding insulation while I was at it.