On a hot day the clear, cool waters of Clear Lake are most inviting, and the lake's sandy beaches are popular. The beach in Wasagaming has changing rooms and washrooms, as well as outdoor showers. And this is a great place for families to swim. It will be packed during the afternoon, particularly on weekends all summer long. But packed by Western Canadian standards, it isn't really that packed.
To escape the crowds, however, there are several quieter swimming areas on the lake. A particularly good one is just past the "Wishing Well" on the lake's north side. However, the lake gets quite deep here, and quickly. This is not a good spot for children or weak swimmers. But it is a quiet and beautiful spot, great for evening swims. During the day it is sometimes used as a scuba-diving site. But you can still swim there anyhow.
There are also swimming areas on Lake Katherine, Lake Audy, and Moon Lake. However, remember that all swimming areas in the park are unsupervised; thus, you swim at your own risk.
Regular lake-swimming safety applies. The water is clear and you can often see the bottom, but it is still wise to always jump in feet first, at least until you've been in the water and gauged the depth, presence of any underwater rocks, or anything like that. It's also advisable to swim with someone else when possible, just in case something happens. Always exercise common sense when swimming.
You can swim in any of the park's lakes, but many have muddy bottoms. I took a dip in Lake Kinosao and found it quite refreshing, even if the bottom was a little muddy. But then I'd just been hiking for several hours.
You'll likely hear a lot about swimmer's itch, a small parasite that causes itching and irritation, which is found in most park waters. Simple precautions of coating your skin with sunscreen or some sort of oil, or rubbing skin vigorously with a towel following swimming, should prevent any complications. Another simple precaution is showering shortly after swimming just to rinse off.
However, swimmer's itch is hit-or-miss anyway. There were several times when I swam where I didn't take any of these precautions and did not get "the itch." Nor did anyone I was with. All this is just to say that all of this talk of swimmer's itch need not keep you out of the water.