Your patron is a noble marid or Olhydra, the evil Prince of Elemental Water.

Patient, haughty, persistent and utterly merciless describe all elemental water patrons. Many easily take offense as well, so any warlock walking this path must tread lightly lest they raise their patron's ire. If you are looking for a kind and beneficent patron look elsewhere, for this type of patron never has those traits. Some are demanding, some may trick you into a pact, but their goals are always overwhelming accumulation.

Special Pact Conditions

These patrons are at odds with the powers of the Elemental Plane of Fire. You are charged with destroying any denizen of that plane you come across. (Killing fire genasi is not required, but may gain you some favor with your patron.)  You are further charged with the freeing of any denizen of the Elemental Plane of water that is bound by mortal magic and the slaughter of those who dare to cast such bindings as summon elemental. While not charged to do so overtly, your patron expects extravagant gifts from you for deigning to deal with such a lowly mortal.

Binding Mark

A large barnacle appears on your head. As you level, the barnacles expand across your face and body till they completely cover you at level 20.

 Pact Boon

Pact of the Chain: your familiar is a ice mephit or a mud mephit.

Pact of the Blade: your pact weapon is coated with elemental ice which does 1d4 additional cold damage on a hit and ignores cold resistance.

Pact of the Tome: your book of shadows manifests as a bowl of the depths; a water-filled bowl from which the water cannot be removed.  When you gaze upon it, you can see your spells written in the ripples in the water.

Pact of the Talisman:  a 4" unmelting crystallized snowflake from the Elemental Plane of Water. Whomever bears the Talisman has resistance to cold damage and advantage on charisma checks when interacting with creatures from the Elemental Plane of Water.

Expanded Spell List

You learn Shape Water as a warlock cantrip which does not count against your warlocks cantrips known. Your Water Patron also 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.

Water Patron Expanded Spells

SPELL LEVEL	SPELLS
1st	create or destroy water, ice knife
2nd	blur, Snilloc's snowball swarm
3rd	sleet storm,tidal wave
4th	ice storm, watery sphere
5th	cone of cold, maelstrom

Gift of the Deep

You learn aquan, the primordial language of water.

In addition, you gain the ability to breathe water as well as air and gain a swimming speed equal to your walking speed.

Furthermore, you are adapted to the most extreme ocean depths and ignore drawbacks caused by a deep underwater environment.

At sixth level you gain the ability to tread across liquid surfaces as if they were solid ground.

Call Elemental

Starting at 6th level, you can call for a water elemental with HD equal to half your level (round up) to come to your aid once between rests. 

The elemental makes a wisdom saving throw vs your spell save dc. If successful, no elemental answers the call.  Otherwise, the elemental appears in the nearest open space close to you, is friendly to you and acts immediately after you in the round. The elemental remains a maximum number of rounds equal to your Warlock spell save dc, or until you thank and dismiss the creature as a free action. 

Frequent usage of this feature, or if called elementals are abused by you or your allies will cause future callings to be less likely to succeed. (Saves vs this feature will be made at advantage.)

Greater Cold Resistance

At 10th level you gain Greater Cold Resistance; you have resistance to cold damage and save vs cold at advantage. If you otherwise have cold resistance, you gain cold immunity instead.

Elemental Form

Starting at 14th level, as an action, You can expend a pact slot to transform into a water elemental. All equipment carried, worn or wielded transforms with you.

You can maintain the transformation without concentration a number of minutes equal to your level on the prime material plane, indefinitely on an elemental plane or a number of rounds equal to your level on any other plane or demi-plane.

You may end the transformation as a free action and it ends automatically if you are killed.

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