For an organism to "evolve to survive" a lot of individual organisms have to fail to survive. The evolution of slight-acid-tolerance in plankton will come at the loss of all the slightly-acid-intolerant plankton. That reduces diversity within species, and obviously wipes out entire species that don't have a self-sustainable subpopulation of acid-tolerant individuals from which to rebuild.

If this happens fast then the die-off will be larger. The loss of diversity also makes those populations less robust to other shocks (what if the acid tolerant individuals within a species also tend to really like cold water, and then oceans warm?).

Yes, we could see evolution happen, but one of the ways evolution acts is through mass extinctions.