If you want serious swimming, check out the, "places to swim," on the USMS website here:
http://www.usms.org/placswim/mapsear...te=CO&units=mi
I think I swam at the Cherry Creek pool ages ago...haven't properly been to Denver (other than through DEN) for almost 20 years.
Most clubs will let you try a workout (some a week). I normally contact them ahead of time, verify time/location, before going.