Source: Dave Calamari of New York state
I remember growing up having to do the pacer test in PE class where we would run across the gym every time we heard a beep, and each minute, the beeps would get closer together. I always thought that it would be a difficult thing to do in the pool.
So... last year I started running it with my swimmers. It's been a great success. We usually run it a few times during the season and track progress.
The interval starts at :36 per 25, and drops 2 seconds every four 25s. When it gets to :20, it starts dropping by 1 second down to :13. I believe the furthest we've had someone get was through the first two @ :15.