I had a Nikko that did exactly that in afternoon sun. Nikko prefers morning sun & afternoon shade. If it's getting full sun, that might be the problem. I think you're doing the right thing by keeping it watered often.

I have a King something or another Hydrangea. I can't remember the name. But it is wilting also. It hasn't bloomed yet this year. Is it too early to move it? Should I move it after it blooms (if it makes it in this south facing position)? Would it recover from a late spring move?

All my hydrangeas wilt in the heat of the day whether they are well watered or not. Even the Nikko that is on the East/North corner of my house wilts in the afternoon. By evening it bounces back.

Well I did end up moving it to the North side of my house near a maple tree. Hopefully it will recover well. The question I have now is should I trim off all of the scorched leaves or let them stay on the shrub?