I think the atmospheric-oceanic diffusion is pretty well known, at least at the surface; shallow water CO2 concentrations appear to follow atmospheric changes on the order of 3-4 years. How CO2 diffuses vertically through the water column is less well known, so we don't know how long or how well the seas will continue to suck CO2 out of the atmosphere. But most of the reefs and fish I'm concerned with are in shallow water anyway.
I'm probably dreaming, given the sheer amount of CO2 that gets shot into the air on an annual basis. But I've reached the conclusion that just reducing or eliminating fossil fuels probably isn't going to be enough, even if it were possible, and that mankind will have to remove CO2 from the atmosphere directly. So far the most efficient means of doing that that I've found is through sequestration of biochar--taking crop waste, which removed a great deal of CO2 from the air during growth, burning it for energy, and capturing the resulting char and gaseous CO2. But using this seawater process as an energy-positive method of extracting and containing CO2 could add to that. If it scales up.