Now you see where the fun starts. Your characters could be sent to this system as part of some Scout mission to determine how the star puts out water like this. Hydrogen in older stars does burn to helium, then beryllium, boron, carbon and ultimately oxygen - so clearly something is causing oxygen from the core to eject into space, and en route that oxygen is picking up hydrogen from the outermost shell of the star.
So the characters would expect the star to have all those other listed elements (curiously enough, though, not lithium - a quirk of nucleosynthesis) in its Fraunhofer lines.
But when their readings confirm the presence of only hydrogen and oxygen combining to form water, and jets with a regular modulation, that is when things start getting a little "hinky," and you have a mystery to solve.
You never know - it really might lead to an experiment of Grandfather's.