Residents are invited to take a dive into the waters of Winthrop Beach benefit Save the Harbor/Save the Bay. On March 23, the organization is holding its third annual Cupid Splash, an event in which people plunge into the waters of beaches along the coast. The Winthrop Beach event will be held at the top of Dolphin Avenue at 12:30 p.m., with registration starting at noon. Save the Harbor/Save the Bay is a nonprofit that advocates for restoring and protecting Boston Harbor and Massachusetts Bay. To sign up for the Winthrop Beach splash, go to www.cupidsplash.com. For more information, call Rose Mazzuchelli, cochairwoman of Friends of Winthrop Beach, at 617-823-5267.