Water Form. The elemental can enter a hostile creature's space and stop there. It can move through a space as narrow as 1 inch wide without squeezing.

Freeze. If the elemental takes cold damage, it partially freezes; its speed is reduced by 20 feet until the end of its next turn.

Innate Spellcasting. The elemental's innate spellcasting ability is Constitution (spell save DC 17, +9 to hit with spell attacks). It can innately cast the following spells, requiring no material components:

At will: shape water, wall of water, acid splash

1/day each: tidal wave, control water, watery sphere

Magic Resistance. The elemental has advantage on saving throws against spells and other magical effects.

Multiattack. The elemental makes three slam attacks. It can replace on of these with a Jet of Water attack.

Slam. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 16 (2d10 + 5) bludgeoning damage.

Jet of Water. Ranged Weapon Attack: +9 to hit, range 80/320 ft., one target. Hit: 18 (2d12 + 5) bludgeoning damage.

Storm Surge (Recharge 6). A surge of water appears around the elemental, flooding in all directions. All creatures in an 40 foot cylinder that is 10 feet tall centered on the water elemental must make a DC 17 strength saving throw. On a failed save, a creature takes 14 (4d6) bludgeoning damage and is both knocked prone and restrained on a failed save. A creature who failed the save can repeat it at the end of each of it's turns, ending the effect on a success and taking another 14 (4d6) bludgeoning damage on a failed save.

At the start of each of the Elemental's turns, the cylinder grows in each direction by 20 feet. Then, each creature in the cylinder must make a DC 17 strength saving throw, with the same effects as above on a failed save.

This effect lasts for a minute, or until the elemental dismisses it (no action required).

Freeze (3/day). If a creature is completely surrounded by water, the elemental can use it's bonus action during it's turn to cause the water round the creature to freeze. When the elemental does this, the creature becomes restrained. At the end of each of the creature's turns, it can make a DC 17 strength saving throw, ending the effect on a success. The ice also has 20 hit point, vulnerability to bludgeoning and fire damage, and immunity to cold damage. Reducing the ice to 0 hp frees the creature.