This looks like sunscald.  Keep it well watered.  You can even spray it off to cool it down, if you like.  Hydrangeas can handle full sun, but they may have put out too much tender growth this spring because of the excessive rain. Do not overfertilize, which can encourage tender growth that can't withstand conditions. 

If this happens repeatedly, every year, it may be a variety of hydrangea that doesn't handle heat well and you may want to more it into a little more shade (especially in the afternoon) or replace with a tougher variety. 

A hydrangea in a pot or on a deck will be subject to even more heat stress than a shrub planted in the ground.

ECN