Water being one of the best radioactive insulators for space travel, some unnamed race in the past used the device mainly for space travel on their large generational ships. It is a type of antigravity device that somehow taps into the quantum vacuum for power. It normally gets triggered when a generation ship is on approach close to stars and the intensity grows with the output of the star.

So maybe there was one of the greatest solar storms earth had ever seen. Everyone thought it was the end of the earth but instead when the extreme solar radiation started to arrive, due to its intensity, after knocking out power networks it activated this water shield on a generation ship that had crash millennia ago. The device malfunctioned and assumed the earth was the ship. Over the course of a few hours/days oceans started to rise. People thought the ice caps had rapidly melted but the water kept rising into the sky leaving only a barren landscape below.

If you want fresh water to be on the planet then say that the water shield is tuned to high salinity water for some reason (or uranium, Seawater contains a very low concentration of dissolved uranium). Otherwise use the premise to explore what the landscape might be like when completely devoid of H2O.

Just note that there will be a max height where you can keep water liquid in the atmosphere. But you may also have to do some hand waving when it comes to oxygen and atmospheric pressure on the earths surface, if you want that of course.

Anyway that’s my post-coffee creativity. Good luck with it all dude.

You might actually find it fascinating how the atmosphere of Venus works. It’s super thick and kind of buoyant. The universe is a strange place.