Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
It's not specified to be. The difference is between the S0/S1 and S2/S3, where the brightness is 1000 nits.

Just want to add The first Generation Apple Watch and Series 1 had 450 Nits of brightness compared to increase of a 1000 Nits currently for the Series 3.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.