pf2e.fyi
From Pathfinder War of Immortals, which isn't enabled.

Arms That Cut the Waves

FEAT 2

River, lake, sea, or ocean, the water calls to you, and you don't fear its depths. When you attempt an Athletics check to Swim, you can spend a Mythic Point to attempt the check at mythic proficiency. When you would run out of air and begin suffocating, you can instead spend a Mythic Point to hold your breath for 10 more rounds.

Pathfinder War of ImmortalsORC