Well in my case, Apple is honoring the warranty after the watch died during its very first lap swim session. In regards to swim mode, starting a swim workout disable the touch screen so that you don’t get false touches from the water. Then when you wake the watch with the crown it expels water from the speaker vents to prevent any long-term problems. However, neither of these things directly impact the short-term water resistance of the device. In other words, it’s not like you have to put the watch in a certain mode for it to be water resistant. In my case, I did have a swim workout session started before going in the water, for what it’s worth.