From Pathfinder Lost Omens High Seas, which isn't enabled.

Aquatic Passage

FEAT 6

Prerequisites Marine Marauder Dedication

You can move through water as easily as you walk on land. You ignore non-magical difficult terrain while underwater and from water-like terrain like swamps or slippery liquids. Moving up or down while underwater is still difficult terrain for you.

Pathfinder Lost Omens High SeasORC