I believe it’s quite a recent feature that Garmin implemented:
When you are swimming, the watch will alert (tone and vibe) whenever you should be at the wall and turning, in order to stay on your target pace. For example, if your target pace is 2:00 /100 m and you are swimming in a 25 m pool, the alert will sound every 30 seconds.
Which is quite handy for adjusting the pace based and it’s make it visible on the main screen which is quite handy during workout. At the moment it’s not something that is integrated with Athletica as it set the pace as a note (I believe).