In July, I escape the southern heat and head to New Hampshire and Northern Michigan. But mostly lakes and not swimming pools (although there are some).

Harbor Springs/Petoskey/Traverse City in Michigan are all very nice in July and not far from Detroit (Delta has a non stop from LHR). It is safe, and very popular in July. I was in Harbor Springs this summer in July and the weather was wonderful. We took a day trip to Mackinac Island and it was just a really nice visit.