This lake is absolutely amazing! I've visited multiple times and I can't wait to go back again. The range of water activities available will keep you entertained, and there are also several floating docks where you can just unwind and relax. If you ever get the opportunity, I highly recommend visiting this place. The admission fee is only €7 (€5 after 13:00)!
The place is clean and offers a beautiful panoramic view, but it was extremely crowded when we visited, leaving us with nowhere to sit :/. It must be really nice on less crowded days!
The Wolfgangsee beach is the ultimate destination. It's absolutely free during the late season.
The public beach by the road with convenient parking nearby is a great spot for swimming in St. Gilgen. Highly recommend!