I’m afraid it’s actually unrealistic, due to the counter-intuitive nature of calcium carbonate chemistry in seawater. Precipitating CaCO3 from seawater actually lowers the pH (makes it more acidic), which in turn tends to drive CO2 back out into the atmosphere.
A quick search found me this reference: Part 1: Seawater carbonate chemistry by Andrew G. Dickson, which is a little technical, but does cover the subject (as well as much more).
The notion of floating PV on the ocean seems feasible to me, certainly floating it on still fresh water is already cheaper and more efficient than ground-based PV.
Over ocean, there are several complications, including waves and salt spray, along with the fact that, unlike fresh water, you can’t simply pump the ambient water over the panels to clean them. And seawater, much less partly evaporated salt spray, is very corrosive to many materials.
All of these are perfectly solvable problems, though (IMO). And with cost reductions due to Wright’s “Law” (also called Learning Curve) which relates cost of manufacture to deployed volume, it could almost certainly become cost-effective.