MM'14 p.333, MM'25 p.365
Compression. The octopus can move through a space as narrow as 1 inch without expending extra movement to do so.
Hold Breath. While out of water, the octopus can hold its breath for 30 minutes.
Underwater Camouflage. The octopus has advantage on Dexterity (Stealth) checks made while underwater.
Water Breathing. The octopus can breathe only underwater.
Tentacles. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 1 Bludgeoning damage, and the target is grappled (escape DC 10). Until this grapple ends, the octopus can't use its tentacles on another target.
Ink Cloud (1/Rest). The octopus releases ink that fills a 5-foot Cube centered on itself. The Cube is Heavily Obscured for 1 minute or until a strong current or similar effect disperses the ink. After releasing the ink, the octopus can use the Dash action as a bonus action.
Underwater