The summer may be nearly over, but shark sightings are not. Several beachgoers reported seeing a shark in the waters off Winthrop at Yirrell Beach and Broadband Beach around 3:30 p.m. yesterday, said Joe Mara, deputy harbormaster for Winthrop. The harbormaster’s office sent specialists to determine what kind of shark, if any, is frequenting the waters. The Winthrop harbormaster’s office was informed that there was a possible great white shark sighting in Dorchester Bay yesterday morning. There have been great white shark sightings throughout the summer in Chatham.