In all fairness to smart watches, mine is what turned me on to regularly checking the UV index.

Can’t you just do that on your phone? Surely if the UV is high, you just plan accordingly for the day? Sunscreen, wide brim hat, stick to the shade where possible, etc. I can’t imagine what benefit constantly checking the UV on your watch gives you. Even if it did happen to fluctuate for some reason, you would be wasting so much time constantly ducking in and out depending on what your watch says at any given moment.