Whenever I'm at Tempe Beach Park, 99% of the time it's for a triathlon or open water swimming event.
The park itself is really pretty, with nice facilities that are usually unlocked and stocked with TP (but I always bring my own just in case).
The water in Tempe Town Lake is really gross. If you're swimming in it (during a legally sanctioned event! Swimming in it any time is illegal), don't drink it. You'll get sick. Trust me on this one.