pf2e.fyi
From Pathfinder Howl of the Wild, which isn't enabled.

Riptide Mount

FEAT 5
Activity 1 action

Requirements You are underwater and adjacent to a creature that is at least one size larger than you, has a swim Speed, and is willing to be your mount.

After working with aquatic beasts of burden for years, you have learned how to easily mount and dismount them. You Mount the creature and Command an Animal to issue it an order of your choice.

Pathfinder Howl of the WildORC