Lake Burley Griffin is not really a lake you'd swim in (I know some people do it and they're free from disease), however if you're into boating or would like to see a different side of Canberra, why not consider hiring a boat at the Canberra Yacht Club!
There are a couple of boats that you can hire for different amounts of time (an hour is generally enough if you're a bit of a novice like me), and it costs around $40 an hour. While you're at it, why not check out the BBQ facilities at Springbank Island!
Don't forget to wear your life jacket.
There's lots of room to park your car and a bistro/bar at the Yacht Club for after water drinks. A fish and chip shop out the back has also proved popular.