Depth Lurker

Most who drown will be forgotten and fade. however a handful of the drowned will be exposed to the ocean's mystery and will be revived as the vengeance from the depths, they are usually able only to remember those who drowned them and will seek vengeance.

Altered by the depths

Starting at 3rd level, you gain a swimming speed of 40 feet and you can breath water and air. when completely submerged water in dim light or darkness, or when in rain or fog you can turn invisible  as a bonus action.

Eyes of the drowned

When you choose this archetype at 3rd level, you have an advantage on wisdom (perception), intelligence (history) and intelligence (investigation) checks that are meant to find information about underwater objects, places and creatures. for you are familiar with the depths. you also gain darkvision if you don't have it already, and a 30 feet blindsight when in water, rain or fog that applies only to what's inside the same water as you. 

Vengeance from the Seas

Starting at 9th level, you have the ability to bring a piece of the sea with you, to bring your enemies to your playground. You can cast a 5th level create water without expending any material and verbal components. can be cast an amount of time equal to your dexterity modifier.
all expended uses recover on a long rest.

One with water

By 13th level, you gained more freedom in water and can for a limited degree swim through air. you can cast misty step as a bonus action to teleport from one source of water to another. you can as well use it freely in fog and rain. additionally you can cast etherealness targeting yourself once per long rest, and use your swimming speed for moving without movement penalty for up and down movement.

Shark attack

When you reach 17th level, you learn how to attack and disappear like a shark. When in water, rain or fog: When you use your sneak attack, you can as part of your action turn invisible and move up to your movement speed towards another enemy and attack him with sneak attack as well. can be chained for up to 3 enemies total.