GPS does not work underwater, so depending on the condtions, waves, and your swim style and speed, the watch may have problems getting the GPS signal when above the surface just for a brief time. Basically it will not work at all on your wrist with breaststroke, or with any other style where the arm with the watch does not stay above the surface long enough.
You can improve the signal quality, if you wait couple of minutes after acquiring the satellites (so called GPS Soak Time), so that it gets the positions of the maximum of the visible satellites. Do the same when you see GPS not working right (distance not progressing, or jumping suddenly) while you are swimming - stop and wait a for a minute with the arm raised to get a better signal.
Enabling GPS+Glonass or GPS+Galileo in the setting of the Open Water Activity, makes the signal-acquiring faster too.
And finaly, if nothing helps, put the watch simply under your swim cap. That's what I do when I decide to swim breaststroke.
