Monk Subclass

An aasimar makes fluid movements and causes the water in the air to condense into a cloud, providing cover for his allies.

A dark-skinned human makes a whipping motion with her hand, causing the water to whip out like a monster, slicing her opponent.

A tiefling pulls his arms down and calms the seas that would have otherwise capsized their boat.

A Water Elementalist is a monk who personifies the adaptability and changing nature of the water. These monks extend their Ki from their bodies into nearby bodies of water, even the water in the air, and manipulates it, allowing for great feats of power.

One With the Current

At the 3rd level, you have learned how to use your Ki to affect the water around you. You gain the following:

• You can breathe underwater and gain a swim speed equal to your regular speed.
• Cast the Shape Water cantrip as a bonus action and can cast Absorb Elements at the cost of 1 Ki, but only against Cold Damage.
• When you make an Unarmed Strike you can decide to draw water from a nearby source (including the air if possible) to make a ranged attack. These attacks have a range of 30 feet, are magical, and deal slashing or bludgeoning damage equal to your Martial Arts Damage + your Wisdom modifier. When you use the Attack action on your turn to use this special attack, you can spend 1 ki point to make two additional attacks with it as a bonus action like you would for Flurry of Blows.

Healing water

You have a basic idea of how to heal using water. You can cast healing hands for 2 ki points. You can spend extra ki points equal to half your monk level rounded down to cast it at higher levels. Additionally, if you heal a person in a pool of sacred water(determined by dm) you heal the max amount.

Additionally, you can stabilize a target with your water.

Student of water.

At 6th level, you have learned to alter the state of the water you manipulate with great skill. Anytime you manipulate water you can freeze it as a free action. If you deal damage with the water you can choose for the attack to deal cold damage instead of the attack's normal damage type. In addition, after you take the Attack action you can spend 2 Ki to cast the Ice and water spells as an action. (seen below) You can have a number of these prepared equal to double your wisdom score.

Cantrips: frostbite, ray of frost, shape water 

1st: create or destroy water, fog cloud, ice knife

2nd:  Snilloc's snowball swarm 

3rd:  sleet storm, tidal wave, wall of water, water breathing, water walk

4th: control water, ice storm, watery sphere

5th: maelstrom

6th: investiture of ice, wall of ice

8th: tsunami

Ever-Changing Mantle

At 11th level, as an action, you can spend 2 Ki points to draw water into the air, where it hovers in your space for 10 minutes or until your concentration ends, as if you were concentrating on a spell. While this water is surrounding you, you gain half cover and can expel some of it at a point you choose within 120 feet of you as a bonus action. Once the water reaches its destination or strikes a solid surface, it explodes, forcing all creatures within 5 feet of where it explodes to make a Strength saving throw or be knocked prone and take bludgeoning damage equal to 1d6 + your Wisdom modifier. You have 2 uses of this feature and can expel either one or two uses as a single bonus action. You can increase your number of uses by spending up to 3 Ki, gaining 2 uses for every Ki. Once you expend your last use of this action, the floating mantle dissipates.

Additionally, you have learned how to manipulate the water around you with great precision. Whenever you use the Shape Water cantrip if you haven't moved up to your full speed this turn you can forgo part of your own movement to move the water that you manipulate with this spell. For every 5 feet of movement, you forgo you can move the water an additional 5 feet.

Blessing of the Moon Spirit

When you reach the 17th level you gain one or the other depending on your alignment 

If you are of good alignment you have mastered the art of pure water bending. You can recover ki during meditation while meditating in a pool of naturally occurring water.

If you are of Evil alignment you have learned to bend other people's blood to control them. You can spend 3 ki points to cast hold person(at 4th lv), 5 ki to cast hold monster, or 10 ki to cast power word kill (once per long rest). 

If you are of neutral alignment go with which one makes more sense to your character.