...So we have a planet that has lower gravity than Earth, low enough for people to float/fly in the sky (so it does have an atmosphere- like Earths), with an ocean that is held together with the use of nano-magnets evenly spread throughout (the ocean covers the planet).
But how do we keep the place from evaporating into space or freezing over?
I think that the atmosphere itself will keep it from evaporating, and I think that having it close to the sun will keep it warm so it doesn't freeze (the planet could be somewhere a bit farther out than Earth is to our sun, but not as far as Mars, or maybe it needs to be closer?).
This planet is artificial so we can literally build it anywhere to achieve our results. I like to believe that its placement in the solar system (again assuming a sun like ours) solves the problem, but I might be missing other aspects.
I also accept that the place is probably not stable, but I'm hoping for a scenario that could give us maybe a million years or so before falling apart.