That may be true, but I've done it with all my watches - sea swims. They have an open water swim exercise. One can expose the watch, but should rinse it in clear water to wash off the salt afterwards. It's more sand than salt that would be an issue, not that salt can't do its damage.
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You can add the exercise yourself. You won't get the same metrics as the preconfigured exercises, but it will keep track of your vitals and calories burned. Go to the exercise tile and tap the dot menu, tap add new exercise then tap the plus symbol and choose create new exercise. Name and set up your new exercise. I find it interesting, though, that they have kitesurfing, windsurfing and water skiing, but not water surfing.
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