Montreal is an island, and while it is not a beach destination, there are a handful of spots where you can swim, sunbathe, and cool off during the hot summer months.
Plage de l'Ile Notre-Dame (also called Plage Jean-Dore) is a man-made beach on the man-made island that was built for Expo 67. The lake is filtered and cleaner than the surrounding river water, and the shallow depth (about 1.5 meters) makes it safe for families. Lifeguards are on duty from mid-June through August. You can rent paddleboats and there are restaurants nearby. Access is easy by metro -- exit at Jean-Drapeau station. The nearby Jardin des Floralies is worth a stroll if you tire of the sand.
Cap Saint-Jacques is the most natural option, located in a nature park in the northwest corner of the island alongside Lac des Deux Montagnes. The beach is less crowded than the downtown options, and the park offers kayaking, canoeing, and hiking trails. Admission to the park is free. Getting there without a car requires some effort -- metro to Ste-Marie station followed by a bus or taxi.
Oka Beach is technically outside Montreal, about 45 minutes west in the Parc national d'Oka, but it is the best beach within easy reach of the city. A long, sandy stretch with good facilities, lifeguards, and equipment rental for windsurfing, kayaking, and pedal boating. The park is open year-round but the beach operates from June to early September. Worth the drive on a hot day.
Verdun Beach (Plage de Verdun) opened in 2019 along the St. Lawrence in the Verdun borough, making it the first urban river beach in the city in decades. It is small and can get crowded, but it is free and easily accessible by metro.