The effect of CO2 concentration on the pH of the ocean
Increasing CO2 concentration in the atmosphere is leading to an increasing concentration of dissolved CO2 in sea-water. The dissolved CO2 then reacts with the water to create carbonic acid, decreasing the pH as recognised in the question.

The solubility of CO2 in water does decrease with increasing temperatures, so the maximum amount of CO2 that can be dissolved is being decreased as global temperatures rise, but the oceans are not yet saturated. There is a some further discussion and references om this page by NASA: The Oceans Carbon Balance. 

Should global temperatures reach a point at which the oceans are saturated and start to release CO2 that they have been absorbing since the start of the industrial revolution, there would be very serious consequences for us all. This is, naturally, the subject of much ongoing research, e.g. Ocean-Atmosphere CO2 Exchange.