With the new Apple Watch Ultra, and WatchOS 9.1, is possible to use the new Action Button to start Waterspeed and immediately Start a new activity, Pause, Stop, or Start a Countdown (you need to setup the countdown activated from the Settings).

For Countdown: after starting, if press the Action button again, it will perform 'Sync -'.

If the activity is started, press Action Button and Side button to Pause.

If the activity is Paused, press Action Button to Resume or Action Button and Side button to Stop.

Be sure to configure the Action Button in the System Settings of your Watch: