Entity: Sailfish Merfolk

Source Howl of the Wild pg. 36 ^2.1

The lower half of your body resembles a powerful sailfish, complete with a large dorsal fin. Using this fin, you can move through the water faster and leap farther. Your swim Speed increases to 30 feet. When you attempt a High Jump or Long Jump, you gain a +1 circumstance bonus to the Athletics check, and you can Swim instead of Striding before attempting the jump.