Aquan Dependency. The undine regains 15 Hit Points at the start of its turn if it has at least 1 Hit Point and is within 20 feet of a body of water, but takes 5 Necrotic damage when it starts it's turn further away than 20 feet from a body of water.

Water Form. The undine can enter an enemy's space and stop there. It can move through a space as narrow as 1 inch without expending extra movement to do so.

Freeze. If the undine takes Cold damage, its Speed decreases by 20 feet until the end of its next turn.

Multiattack. The undine makes four water jet attacks.

Water Jet. Ranged Weapon Attack : +9 to hit, range 90 ft., one target. Hit : 9 (1d6 + 6) Piercing damage and 3 (1d6) Cold damage. If the undine is more than 20 feet away from a body of water, it takes 7 (2d6) Necrotic damage as it drains it's body of water to attack.

Torrent (Recharge 5-6). Dexterity Saving Throw : DC 18, each creature in a 60-foot-long, 5-foot-wide Line. Failure : 14 (4d6) Bludgeoning damage and 14 (4d6) Cold damage. If the target is a Huge or smaller creature, it is pushed up to 10 feet straight away from the undine and has the Prone condition. Success : Half damage only. Success or Failure : If the undine is more than 20 feet away from a body of water, it takes 28 (8d6) Necrotic damage as it drains it's body of water to attack. 

Dissolve. If it is fully submerged in water, the undine becomes Invisible until  the start of it's next turn.