At this time the other answers either miss or gloss over the core of the reason why such a structure could not be possible [by natural means]:
Fission and materials filtering.
With the right star and enough H20 in the correct initial orbits, there isn't a lot to actually stop such a structure from existing for a decent chunk of time on a geological timescale.
Sure, the structure is going to fail eventually, and fairly quickly on astronomical timescales, but in the grand scheme of things our own solar system is going to fail relatively fast compared to the overarching universe... [And we seem to be doing somewhat okay for the time being...]
The point however is that despite how implausible it is to have that much material in that kind of orbit, the far more implausible point is getting that material in the first place...
So the core of our real issue here is all the 'extra' elements that get created along the way when you start off with Hydrogen and run things along well enough to come up with notable amounts of oxygen: There is 'other stuff' in there, which will heavily contaminate our 'water source' even if we otherwise have perfect conditions for creating the 'temporary' water ring.
You would need to have had enough 'free' hydrogen and oxygen in the system for all the water required, while not having a volume of stuff heavier than oxygen to group up and form cores that interfere with your water ring, and having somehow filtered the bulk of everything between Hydrogen and Oxygen out of that region... All without having displayed the target water from its required initial formation motions.
- I can napkin math a star with a water ring that exists for a time, but I'm not seeing any kind of a starting point to napkin math anything close to a filtering mechanic short of "god/aliens did it".