Very limited by the GPS coverage while under water.  I have found if I try to get sections one arm swimming or better yet of swimming on my back (with my gps arm 'resting' for at least a couple seconds to reconnect/sync to GPS above water).  Easy enough to do it when I am snorkeling.
Otherwise some people attach the GPS to a float/safety-bouy  

When I turn direction I usually hold my garmin swim above water for 5 to 10 seconds. Mostly that is enough to regain gps reception. Nicks method of using a swimming style where the garmin is mostly above water works too.