Stroke rate is an interesting parameter to measure while swimming. Swimtraxx has developed a device that offers to do so besides tracking many other swim specific parameters.
With Swimtraxx One you can keep track of the frequency of each of your strokes. Stroke by stroke. After the data is measured in real time you consult your workout details in the Swimtraxx app.
Check out https://www.swimtraxx.com/#features for more info!