...an empirically-based objection to the theoretical existence of
what some of you are calling "Water-Giants"; I'm assuming you're thinking
that you could have a Jupiter-mass, or Jupiter-diameter world made
mostly of water.
H20 is about 88% oxygen by mass, so if a Jupiter-mass world
were to be formed/created out of planetary nebular material, that
nebular material would have to be at least 80% percent oxygen by mass.,
with the rest being mostly hydrogen [very little helium] and a few percentage points belonging to the heavier elements [carbon, magnesium, silicon, etc]

Highly unlikely, to say the least...[No such oxygen-saturated nebulae, planetary or otherwise, has ever been detected]