This is the GARMIN QuickFit Bungee Band for Descent

Features:

Strap, secure and dive with the 20, 22 & 26 mm bungee mount. Designed for divers, the band is elastic enough for comfortable above-surface wear while still remaining snug at depth. Like to wear your watch on the back of your hand or farther up your forearm? Just attach the mount, and the redundant shock cord system easily keeps your device in place. Even if you’re not a diver and simply want to explore, secure the mount over heavy jackets, and rest assured your device will stay with you wherever you go.