I found this today while doing research on tempo trainers for the pool. Very simple and truthful advice -
Swimming is the OPPOSITE of running. In running your cadence remains virtually constant, yet your stride length increases as your speed increases. In swimming, your stroke rate increases as your speed increases, and your distance per stroke remains constant.
Do a set of hundreds (say 10) where you count the total number of strokes per hundred. Keep that number constant and descend your pace. Once you get to the point where you can't descend anymore or where you have to start to take more strokes, then you have found the point where you breakdown. Do lots of training at this pace. You will get faster.
Many people I see that swim slowly are literally swimming too slowly. Their turnover rate is like a film stuck in slow motion. If you simply pick up your cadence, you will be very pleased with the results. Fast triathletes swim at a cadence of 60 to 75 strokes per minute. Measure youself and give it a try!