When there is an increase in temperature, how is the pH of the ocean affected? (Question is related to ocean acidification)
You have to compare the size of these counteracting effects.  

The temperature increase slightly decreases the acidification caused by increased carbon dioxide partial pressure.

The shift of reaction constant is minimal.

From the Wikipedia carbon dioxide data page, Water solubility section, we can see for temperature  15 resp 17 $^\circ$C V/V solubility 1.019 resp. 0.956. That is 6.2% lower solubility.

Increased partial $\ce{CO2}$ pressure from 0.03 to 0.04% gives 33%, what is significantly more than by 6.2% decreased solubility.

Aside of that, higher $\ce{CO2}$ concentration and lower sea water $\mathrm{pH}$ are already observed. 

This is directly affecting carbonate related equilibriums, particularly in context of forming of mollusk shells.

Some mollusks are already observed  having troubles in forming aragonite - the more soluble form of calcium carbonate.