As two actions, you can create a vial of cloudy green fluid that smells of the sea and has a jellyfish-like bubble floating in it. Choose fins or gills as the concoctions’s effect. A creature can drink the contents as an action to gain the chosen effect:
Fins. The creature grows fins and webbing between its fingers and toes; it gains a swimming speed of 5 meters for the duration.
Gills. The creature grows gills; it gains the ability to breathe underwater for the duration. It also retains its normal breathing pattern.
You can expend 1 additional catalyst so the concoction produces both effects.