Answer: It's difficult for me to tell what the problem might be without actually seeing the plant but here are a few things I've seen in the past:
Hydrangeas tend to droop in the hot afternoon sunshine. This does not indicate the need for more water, it's just a defense system the plant uses to avoid dehydration. Usually the leaves look fine in the morning but droop again in the late afternoon. The second possiblility is that the branches are just too long and unable to support themselves. Fast growth can cause this and there's no way to strengthen the branches other than to support them. You can avoid this problem in the future by not over-feeding. If you'd rather not prune to shorten the branches then you can certainly stake them up.
Best wishes with your hydrangea tree.
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