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I am sorry you have the issue. It is certainly odd it stopped working for open water swimming. If you are still able to get ambient temperature when you remove your watch and check your temperature widget, there is a chance you can correct the behavior as long as it is not an overall hardware failure of that sensor.
Please try the following:
Soft reset your watch by holding down your top left button until your watch shuts off then let go of the button.
If needed, fully reset your watch:
If your issue continues, please reach out to Outdoor Product Support in your region.