Dallas (and the rest of the Metroplex) have a ton of lakes for open water swimming. Do be aware that there is a lot of boat traffic on most of these lakes, so you can't just go across them - stick to the no-wake zones or to protected coves.
Some favorites include
Lake Lewisville - Pier 121
Lake Grapevine - Murrell Park
Lake Lavon - Brockdale Park
Triple Threat Tough (a triathlete training group) puts on coached OWS practices at Lake Lewisville - check out their calendar at http://dfw.triplethreattough.com/groupworkouts/triplethreatcalendar.php and contact them for more information.