Ocean acidification is a growing problem as a result of man-made CO2 emissions. The name is slightly misleading though. Sea water is actually alkaline, averaging around pH 8.2. The introduction of all the CO2 is making the oceans less alkaline rather than more acid but the end result is the same.
The reason the situation is not as bad as it might be, and also why it was freshwater lakes that were being worse affected by acid rain than the seas, is that the salts dissolved in sea-water produce a significant buffering action.
In this experiment pupils, or the teacher, blow air through two solutions, tap water and sea water, both containing universal indicator. The sea water remains neutral for far longer than the tap water dues to the buffering action of the salts it contains.