The Deep One Patron

Your patron is an ancient being who lives in the deepest places of the world. These beings seek to submerge the world in their oceans, or to rule all with their powerful psionics. Whatever your patron, it will seek to use you as an instrument to do this. Perhaps a scout for the beast as it looks through your eyes at the surface world, or the first wave of it's assault. These beings will seek to dominate the surface world with whatever tools it has at it's disposal. 

Some suitable patrons for this pact may be Kakens or leviathans as well as creatures from the elemental plane of water. The Merflok kings and powerful Merman may also seek to create pacts. Hive minds of abberation colonies could also create pacts through their psionics. 

Expanded Spell List

The Deep One lets you choose from an expanded list of spells when you learn a Warlock spell. The following spells are added to the Warlock spell list for you.

1st Level: Create or Destroy Water, Fog Cloud

2nd Level: Detect thoughts, Phantasmal force

3rd Level: Water Breathing, Water Walk

4th Level: Evard's Black Tentacles, Control Water

5th Level: Cone of Cold, Dominate Person

Grasp of the Depths

Starting at 1st level,  your patron grants you the ability to summon dark tendrils to grasp and bind your enemies. Whenever you hit a creature with a weapon or spell attack you can force the creature to make a Strength saving throw against your spell save DC or be restrained as tendrils of water form around it. These tendrils last for 1 minute or until the creature escapes the tendrils. A creature can use it's action to attempt to escape the tendrils, by re-making the Strength saving throw, ending the effect on a success. Once you use this feature, you cannot use it again until you finish a short or long rest. 

Liquid Movement

At 6th level you begin to take on the form of water. You can maneuver between any space no smaller than 1 inch without squeezing. Moving like this is treated as difficult terrain for you. Additionally, you gain a swimming speed equal to your walking speed and can breathe underwater. 

Submerge the Doomed

Beginning at 10th Level, your patron has now allowed you to conjure large waves to drown your foes. As an action, you can summon a wave that cover a 60 foot cone originating from you. All creatures within this cone takes 4d6 Cold damage and must make a Strength Saving throw against your spell save DC or take 6d6 Bludgeoning damage and be pushed 20 feet away, up to the edge of the cone and knocked prone. A creature that succeeds on the saving throw takes half the damage and isn't knocked prone. All non-magical flames in this area are extinguished. 

Once you use this feature, you can't use it again until you finish a short or long rest. 

Wrath of the Tides

At 14th Level, you can now summon 8 tentacles and tendrils to grasp your foes. As an action, you can target up to 8 creatures within 30 feet of you. Each creature of your choice must succeed on a Strength saving throw, On a fail, you choose one of the options for the creature. A creature can only be affected by 1 effect at a time. Once an effect ends, the tentacle vanishes and is unusable. These tentacles last for 1 minute or until all the tentacles are used. Any tentacles not in use after the effect has been activated can be controlled as a bonus action. 

-Grasping tentacle: The creature is restrained on a fail. The creature can spend it's action to re-roll the saving throw, ending the effect on a success. Once this effect is ended, the tentacle vanishes, becoming unusable. 

-Shoving tentacle: The creature is moved 10 feet away from its space in a direction of your choice. The tentacle then vanishes and becomes unusable. 

-Slamming tentacle: The creature takes 2d6 + your Charisma modifier as bludgeoning damage. The tentacle then vanishes and is unusable. 

Once  you use this feature, you cannot use it again until you finish a long rest.