As part of an ongoing effort to update a planetary classification system for sci-fi settings, I'm trying to nail down if an old idea for an exotic type of life-bearing world is actually plausible and I'm not sure how the various factors balance out. The idea is a "Wet Venus" sort of world, with much higher air pressure than Earth, made mostly of CO2, with lesser amounts of nitrogen and oxygen. Oxygen is a lifting gas in this atmosphere, helping an aerial ecology form in the upper layers of the atmosphere, leaving the surface dark under multiple cloud layers.
The trick I'm having trouble figuring out is how plausible is the CO2 remaining in the presence of an ocean of water. On the one hand, Henry's Law states that there will come a saturation point where CO2 is balanced between air and ocean. On the other hand, CO2 dissolves into an acid and most research I've seen on Earth's ancient atmosphere seems to say that the oceans got rid of the primal mostly-CO2 atmosphere. On the gripping hand (for the sci-fi fans) an acidic ocean would also make the formation of things like limestone not a thing, which would help keep CO2 in the air.
Which way does this balance out? The dense CO2 really helps make floating life doable, but is that plausible? Would having the world be more volcanic be needed?