I wrote a six article series on the Divemaster program for my LDS's newsletter. I give it to all of my Candidates to help them through the process.
Here's a snippet from the Swim section:
The first part of this is the one most candidates worry about, but dont! Its really not that hard to get through: Swim Evaluations. You are required to do 4 individual swim tests, and you must score at least 12 points in total (all 4 swims together) in order to be successful. You earn anywhere from 1 to 5 points on each swim, depending on your performance. The first is the 400 yard swim. Simply swim 400 yards, using any stroke you wish, or combination of strokes, without stopping and without any swimming aids. This is just swimming laps in the pool, or a designated distance in open water. Swim hard, swim slow, alternate between the two or however you like, but finish before 10 minutes and earn a 3. The next evaluation is a 15 minute tread. In any water too deep to stand in, tread water for 15 minutes, with your hands out of the water for the last 2. Do this, and its an easy 5! Remember, its just hands out of the water for the last 2 minutes so dont make it harder than it has to be. The 800 yard snorkel swim is next. You will probably do this on open water unless you have easy access to an Olympic size pool for laps. You use your mask, fins, and snorkel. The key to this one is a great snorkel. A large bore without any restrictions (like valves that close when you submerge) will serve you well. Finish this swim in less than 17 minutes and you have another 3. The final evaluation is a Tired Diver Tow. Pick a small diver and use your choice of tows: pull or push. The push is for most the fastest way to go. Go 100 yards in less than 4 minutes and youve got another 3. For all of these swims, the faster you go the better your score and you can earn 4 or 5 points in each if youre in good enough shape, but what Ive listed above is an average and will get you the required 12 points you need.