Please remember that Swimmo is equipped with a unique screen and you need to tap it harder than you would a normal touchscreen on a smartphone!
Tap the screen harder than on a smartphone to wake up the display or select an option.
Rotate (flick) your wrist away or toward you to change options when the arrows appear.
As opposed to all other functions, pausing a workout requires a double tap.
You can find a video showing the basic navigation here.