Question about Garmin Venu 3

i recommend rinsing the watch with normal water after swimming

If you use the watch as intended, the warranty cannot be invalidated.
Swimming is an explicit function or available during training, so the warranty cannot be invalidated by this, which would be contradictory.
I have used it several times while swimming and it still works perfectly.

The watch has training functions specifically for swimming. There is no problem if you use it in the pool or sea for swimming