Bay Crab

Amphibious. The crab can breathe air and water.

Brittle Carapace. When the Bay Crab drops below half of its maximum hit points its shell begins to fracture and break. Its AC drops by 2.

Brined Nature. When below half of its maximum hit points, when the crab takes damage of any type but psychic, each creature within 5 feet of the crab takes 9 (2d8) acid damage.

Damage Threshold. If crab takes any type of damage besides physic that is equal to our less than 15 points of damage, the crab instead takes no damage. 

Claw. Melee Weapon Attack: +9 to hit, reach 15 ft., one target. Hit: 27 (4d10 + 5) bludgeoning damage. Creatures within 5ft of the target must make a dexterity saving throw (save DC 15) or take 14 (2d10+5) bludgeoning damage.

Water Jet. The crab shoots water out of its body at high speed in a 60ft line that is 5ft wide. Each creature in the line must make a dexterity or intelligence saving throw taking 24(8d6) force damage and be knocked back 10ft on a failed save. On a successful save the creature takes half damage and is not pushed.