It sounds as though you've gotten the answer from others.
Having recently taken DIR-F (it's a great class) -- and also being someone who dabbles in easy freediving frequently -- I'd pass on the following tip for completing this successfully. I've noticed that many scuba divers who do this test overexert -- kicking madly and flailing like a jackrabbit. Some have had a challenging time completing a 50-foot swim. However, the most important thing in any breathhold swimming is to keep your exertion level low, stay very calm and meditative, and try to maximize your forward motion for amount of muscle motion. Take one stroke at a time and let yourself coast as long as possible. Here's a video of David Lee, a world competitor in "dynamic without fins" (what the competitive freedivers call this kind of swim), getting across 25 meters in 21 seconds with three kick cycles):
http://www.apneablue.com/videos/david_25m ucb pass.wmv
Although he does a combination of breaststroke motions with a little dolphin kick thrown in, when I do this kind of swimming I find that I get 75% of my motion from the breaststroke arm motions alone.
Also, if you have trouble completing it, try a retest within, say, 5 minutes of the first try. Freedivers usually warm up by doing some breathhold practice (whether swimming or "static") before their swim that counts.
Hope this might help a little. Have a great time in the class, it really helped me a lot.
P.S. Two important addendums. (1) Take no more than 3 to 4 deep breaths before starting (so as to avoid problems from excessive hyperventilation). (2) Never practice breathhold swimming unless under the direct supervision of a buddy. Although the chances of blacking out in a swim this long are fairly remote, people can and do die in 1-2 feet of water in pools (including freedivers who a lifeguard was supposed to be "keeping an eye on").
