https://wccftech.com/how-to-unlock-aloys-diving-mask-in-horizon-forbidden-west/

The Diving Mask is one of Aloy's most crucial upgrades to exploring underwater caverns and cauldrons. This strange mouth apparatus was teased in promotional materials ahead of the launch of Horizon Forbidden West and led players to believe that Aloy would be able to take the fight under the waves.

Without the all-important Diving Mask, Aloy's ability to hold her breath under the waters is reduced to roughly thirty to sixty seconds at a time. The developers at Guerrilla have given Aloy a bit of breathing room, no pun intended, to survive a few extra seconds when her oxygen runs low if the player is making an active effort to surface for air. However, once Aloy gets her hands on the Diving Mask, she's able to stay underwater indefinitely for all of your aquatic spelunking needs. After all, rare Greenshine pieces for upgrading the highest tier of gear can often be found where humans can no longer tread.