Keen Smell. The giant nudibranch has advantage on Wisdom (Perception) checks that rely on smell.

Venomous Defense. Any creature within 15 ft. of the blue glaucus must make a DC 13 Dexterity saving throw on its turn, on a failure the blue glaucus can make a stinging cerata attack against the creature as a free action. In addition, creatures attacking the blue glaucus in melee (unless using a reach weapon) must succeed on a DC 13 Dexterity saving throw or be automatically hit by a stinging cerata attack.​

Water Breathing. The blue glaucus can breathe only underwater.​

Waterbound. A  giant nudibranch can only use its ground speed when submerged in water.​

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) piercing damage.

Stinging Cerata. Melee Weapon Attack: +6 to hit, reach 15 ft., one target. Hit: the target must make a DC 13 Constitution saving throw, on a failed save they take 13 (3d6+3) poison damage and are poisoned for 1 hour, on a successful one they take half as much damage and are not poisoned.​