"I'm about halfway through this book and it's very interesting. Is the premise possible though? Jupiter in this story has been discovered to have a global liquid water ocean about 70,000 km below the visible louds. The ocean is laced with sulfur and ammonia but is still essentially a water ocean about 5000 km deep. Enormous gravitational pressures compress the gases in Jupiter's atmosphere into liquid form. The ocean is a relatively thin onion skin layer compared to all of jupiter but is still 500 times deeper than the deepest trenches in Earth's oceans. Is this scientifically feasible?"

I haven't read the book. But H is common and O is common, and H2O is common in this Universe and our solar system.
And I would expect that there is a lot of H2O in Juptier. The fact that we haven't detected significant amount of H2O from Jupiter isn't surprising.
If you were to state that there was little water in jupiter, you would need to explain where all the water which entered Jupiter (from comets) went. Me thinks, it couldn't escape from a planet with a huge Hydrogen atmosphere.