These monks study by the sea or other large bodies of water. Generally, these monks combat pirates or the mostly evil sea races that antagonize them. They specialize in using their ki to become one with water.

I recommend making ki the resource they use instead of spell slots. Cantrip is 0. Ki is based off spell level. For example, a level 3 spell would be 3 ki.

Ebb and Flow

Like water, you both ebb and flow.

Ebb: Starting at level 3, when you use flurry of blows, you get disengage as part of that action.

Flow: Your mastery of ki has granted you the ability to channel your own ki into tangible water. This is no small task, if casting a spell via ki takes the last of your ki points, take one point of exhaustion. Your spells take a number of ki points equal to the spell level. I.E. a level 3 spell is 3 ki points. Wisdom is your spell modifier. 

One with Water

Starting at level 6, you feel more at home in water. Fighting underwater can never grant you disadvantage. Likewise, you gain the ability to breathe water. For all intents and purposes you are amphibious. Moreover, casting your ki spells in water grants a +2 bonus to your DC and 1 additional damage die. If the spell doesn't have a damage die, the effect is amplified by 5 feet. For instance, shape water becomes 10 feet.

Also, starting at level 6, watery tendrils come out of you. Your reach with these tendrils is 10 feet.