Ninja of the Tide

In the halls of Bast, hidden in the northernmost regions of the Gaping Maw, lies a temple dedicated to the Mistress of the Shadows. These halls are the location of the legendary clan of Pirate Ninjas, a somewhat silly-sounding group of marauders who specialize in stealth and the world's various elements. In the land of Einess,  they are known as the reapers of the high seas, slaughtering entire ships with nothing but 3 honed clansmen and a small, camouflaged rowboat. Victorious assassinations usually mean that the ship you worked so hard on getting will be sold or dismantled for parts.
Of the various ninjas, the Ninja of the Tide is the ninja who specializes in water and can use its various techniques to control the very ocean itself. They are able to begin moving water and manipulating the moisture in the air from the beginning. At later levels, they gain the ability to use water for combat, giving them the upper hand in combat. Once they have mastered their respective elements, the Ninja of the Tide fully controls their own techniques, allowing them to become the very element they control.

Gentle Waters

Starting at 3rd level, your calmer, gentler style of stealth gives you the ability to manipulate the power of water with your body's chakra. You can cast the spell Shape Water as a cantrip at will. If you already have the spell Shape Water, you instead increase its range by 30 feet. In addition, if the water you are controlling is seawater, the range of the spell doubles, and you can affect 10 cubic feet of water with the spell. 

Shape of the Sea

When you choose this archetype at 3rd level, you gain the ability to control your breathing within the waters. You can hold your breathe underwater 3X longer than normal, and if you are in salt water, you can breathe indefinitely, and speak common while you are fully submerged.

Lurker of the Abyss

Starting at 9th level, you can now breathe and speak in any form of liquid indefinitely, and have advantage on stealth rolls if you are completely submerged in water that is more than 10 feet deep. Additionally, you gain the spell Water Whip, cast at 2nd level, and can use it an equal amount of times to your proficiency modifier, as well as perform Sneak attacks with it. The opponent must make their  Dexterity saving throw, which is 8 + Your spellcasting modifier (INT) + Your Prof bonus. If failed, the opponent takes the bludgeoning damage and your sneak attack, but does not get pulled 25 feet towards you. If passed, the attack is considered a miss, and the foe does not take half damage.

One with the Water

By 13th level, your skills with the water have grown exponentially. Your movement speed doubles when you are swimming, and you are able to attack with any weapon, including ranged weapons, underwater without having disadvantage on your attacks.

Additionally, when you are performing a sneak attack using the Water Whip feature while you are submerged in water, you can use your ability to instead pull them into the water, where the water can begin to actively suffocate them. Creatures that are your size or smaller need to make a DEX save equal to your Spell Save DC (8+INT+Prof) in order to prevent themselves from going under. When they are underwater, they are considered grappled by the water, and make strength saving throws to resurface with disadvantage. After 1+(con bonus) rounds of them holding their breath underwater they fall unconcious, and if they are in combat, they will die.

Master of Water

When you reach 17th level, you have become a true master of the Tide. Your body is able to enter a state of semi-liquidity. You can use an action to turn yourself translucent, and can show the background of whatever you are through your body. You gain advantage on all stealth checks while you are in this watery form, are immune to fire damage, have resistence to nonmagical piercing, slashing, or bludgeoning damage,  and are vulnerable to Lightning damage. Additionally, while you are in your watery form, you can pass through objects like doors, vents, and windows that water have an opening and can leak through, as long as they were not designed to be leak-proof. This form lasts for 10 minuets. If you end your turn in a space that is occupied by an object, or is too small for you to fit into, you reappear in the closest space you can fit, and immediately take 6d8 bludgeoning damage, You can assume this form an equal amount of times to your INT modifier, and regain uses on a long rest.