Lake Erie is the warmest of the Great Lakes and Sandusky has several swimmable beaches within a short drive of downtown.
Cedar Point Beach — a mile of sand on the south side of the peninsula, included with park admission.
Cedar Point Shores — Cedar Point's separate water park, with wave pools and slides, open from Memorial Day to Labor Day.
East Harbor State Park — 20 minutes east on the Marblehead Peninsula, with a long natural sand beach, picnic areas, and a small campground.
Sandusky Bay itself is shallow and muddy and not suitable for swimming; head out toward the lake proper for usable beach.