Welcome to the wonderful world of wearing a watch. But since you will now continue to wear a watch, the tan lines don't matter because they will be covered.
Pro tip, consider loosing the watch a bit on a sunny day out so it goes up and down the wrist from time to time.
How could Apple not have thought of this? With $200 billion cash, couldn't they figure out how to make a watch out of materials that let UV light through? Steve Jobs would never have let this product go through. For all of Angela Ahrendts' fashion connections, she didn't think about tan lines?