We have estimates for the amount of water in the ocean, and as you can see from the chart above we can observe how much CO2 is locked up in samples.

Extrapolating that to a total isn't rocket science, but it also isn't particularly interesting.

It doesn't matter, how much there is stored. What matters is how much it takes to shift the balance, how fast it's happening and how strongly it affects ecosystems in the ocean.

Those things are much harder to quantify, but evidence so far suggests that it is an extremely fragile system and even the small changes that are already taking place are harmful.

Even if we stopped every process that produces CO2 now, the ocean would slowly continue to absorb a lot of the extra we already pumped into the atmosphere.
It's a huge help in other aspects, actually, but it comes at a cost.