Aquatic Survivor. Has resistance to all damage types, and has AC increases by 2 while submerged in water.

Multiattack. The Underwater Panther makes 4 attacks. One with its tail, one with its bite, and two with its claws.

Swat. Melee Weapon Attack: +10 to hit, reach 5 ft., 1 target. Hit: 30 (5d6 + 5) Slashing damage. 

Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., 1 target. Hit: 60 (10d6 + 10) Piercing damage. 

Tail Spike. RangedWeapon Attack: +10 to hit, range 10/30 ft., 1 target. Hit: 15 (2d8 + 3) Poison damage. If target is struck by this attack, they must make a DC 17 Con Save or be poisoned, and take 1d4 damage at the start of their turn for 10 of their turns or till combat ends.

Healing Water. If submerged in water at the start of its turn, the Underwater Panther can regenerate 2d8 hit point by succeeding a DC 17 Con Save as a bonus action.

Parrying Swat. If an creature makes a melee attack on it and misses, the Underwater Panther may attempt a Swat attack back at them.