Smart swimwear with embedded UV sensors that connect to augmented reality, creating real-time 'eclipse maps' of sun exposure on your body. Point your phone and see exactly where sunscreen has worn off.
You point your phone at your friend after 3 hours on a cruise ship deck and see a glowing red 'eclipse shadow' on their left shoulder and upper back— the app reveals exactly where UV has penetrated their sunscreen, showing the invisible damage in real-time before it becomes a burn.