[spunkets]

"Adding dry ice to the water is a quick way to add more carbonic acid to the system. But the flask should be sealed to increase CO2 in the headspace, so more dissolves in the water over time."

Adding dry ice just increases the CO₂ pressure to 14.7 and an equilibrium conc. will be reached. If the flask is sealed it will most likely explode. Any drift seen on the pH meter is drift in the glass and reference electrodes.

[cogitator]

HUNBLE: read safety comments! It won't explode if you don't use too much dry ice. But the pressure in the flask will increase.

Any drift seen on the pH meter is drift in the glass and reference electrodes.

Have you ever had a taste of soda water? If so, why is it "tart"?

If you put dry ice in an open glass, let it melt (actually sublimate), then taste the water, you'd discover that the water tastes tart -- because of the carbonic acid that dissolves in the water from the dry ice. The pH has to change when this happens. Seawater pH will change less, because of the buffer system. But it will change.

Hunble, this brings up a safety factor. I'd suggest doing this experiment in a plastic soda bottle. Add the dry ice in little pieces, then put a balloon around the neck of the bottle and affix it tightly with a rubber band. The ballon will inflate some, but this will keep the headspace charged with CO2 while greatly reducing the explosion risk.