I upgraded from an S8 to a U2 on launch and have been wearing it since. I recently noticed a red area on my wrist directly under the sensors. It doesn’t hurt, burn, or itch. My skin isn’t raised and feels normal to the touch. If you look closely the skin is “scaley” (and red, of course). Anybody else notice this?
Most likely Contact dermatitis. Not uncommon with watches particularly if worn 24/7. Harmless, not contagious. Try having a break wearing it at night or swapping wrists for a while. It will subside within 3-4 weeks.