If your gong to be in Ann Arbor there are a few places somewhat close. All with in 40 minute drive would be Pinckney State Recreation Area (Silver Lake unit), Island Lake State Recreation Area (Spring Mill Pond). Further out would be Pontiac Lake State Recreation Area. Sterling State Park on Lake Eire in Monroe (not overly thong friendly). There are 3 public pools in Ann Arbor (not sure of their swimwear policy). Send me a PM and I can help out with information about the area.