In the summer month, you can swim in Upper Waterton Lake. The most popular spot for swimming is Emerald Bay. There's also a beach near the Townsite campground that gets windy but can be used when it isn't too hot outside or else you'll find yourself with sore muscles from all those laps!  Alongside the Marquis Hole picnic site along the river, there are plenty more places to cool off and get your mind off things.