I just did mine 2 days ago. It wasn't hard at all. The time limit is 80 minutes which is plenty of time. There are 4 official navy strokes that you will use (and be taught if necessary) in API Water Survival: American Crawl (aka freestyle), Sidestroke, Breaststroke, Elementary Backstroke (aka frogstroke). You can use any combination of these, except the backstroke, on the mile. You will get plenty of practice before hand and instruction if needed. We had 2 fail out of ~35 and they both quit after about 20 minutes, one had just had the flu; everyone else finished with time remaining. They will both get EI for a week or so and try again. If you still have problems they will roll you out of class for a full week of swimming instruction. So don't worry just do the work and don't be afraid to ask for help. But if you want to prepare for it the best thing you can do is be in good cardiovascular health and spend some time in the pool getting comfortable in the water.