This is a Popular Science article from a few months ago, but I came across it last night, found it fascinating.

Rob Verger, Popular Science:

The device has functioned as a swim tracker since it became water-resistant in 2016, but with its latest operating system, it presents a more granular metric: set detection. It knows when you rest at the pool’s edge and then uses that information to divide the workout into sets of laps, showing you how far and long you swam in each, what stroke, and your rest time.

And:

Apple built its own algorithms to discern the stroke of swimmers of all skill levels, as well the calories they’re burning. The process involved gathering data from more than 700 swimmers and over 1,500 swim sessions. And Apple gathered even more data from people swimming in place (in an “endless” pool) while wearing a mask that descended from the ceiling.

And:

The watch uses the gyroscope and accelerometer to track the motion of your strokes, but in open water, it can leverage another sensor: the GPS chip. If you’re swimming in the ocean or a lake, your watch uses that to determine how fast and how far you go. But GPS signals don’t travel through H2O. Luckily, people are likely to do freestyle in open water and for that stroke, your arms regularly break the surface. Apple sets the GPS chip in acquisition mode for the whole swim. It looks for the satellite signal each time your hand rises from the water. “We’re trying to catch it every single time,” says Ron Huang, Apple’s director of engineering for location and motion services.

I absolutely love the effort that Apple puts in to get things like this to work. This is Apple at its best. Read the article. As I said, it’s fascinating.