Tempest Sorcerer

Your ancestry is connected to an aquatic creature or perhaps to the plane of water itself, either way you've always felt a calling to the ocean. 

Soul of the Sea

At 1st level, your tie to the sea grants you the ability to breathe underwater, and you have a swim speed equal to your walking speed. You also are proficient with vehicles (water), tridents and short swords.

 

Curse of the Depths

Once per turn when you cast a spell or cantrip, you can trigger the curse if that spell deals cold/lightning/thunder damage to the target or alters movement. Doing so subjects the target to the appropriate additional effect below (you choose only one effect to use if more than one effect applies):

 

Cold Damage. If the affected target takes cold damage from your spell, the target’s speed is also reduced by 15 feet until the end of your next turn. If the spell already reduces the target’s speed, use whichever reduction is greater, not both.

Lightning/Thunder Damage. If the affected target takes lightning damage from your spell, the target takes additional lightning damage equal to your Charisma modifier.

 

Spell List: 

Cantrip: Shape Water 

1st: Create/Destroy Water, Animal Friendship(aquatic theme only)

3rd: Tasha's Mind Whip, Misty Step

5th: Call Lightning, Tidal Wave

7th: Control Water, Raulothim's Psychic Lance

9th: Maelstrom, Commune

Siren's Call

Starting at 6th level, you have mastered the hypnotic words of a siren, you gain proficiency in the persuasion skill. This becomes expertise if you are already proficient.

As an action you can unleash the charm of a siren onto one creature within 30 ft of you. The creature must make a Wisdom saving throw to resist, DC is equal to your spell save. Creatures who normally have advantage against charm roll normally, unless they are immune to charm(they are then unaffected). On a failed save, they treat you as a trusted and old friend, this lasts up to an hour. This charm is broken if you attack them or an ally. Once the hour is up or they succeed their saving throw, they have no clue you tried to charm them. You can use this feature once per long rest.

Aquatic Adaptation

Once you reach 6th level your natural powers have begun to flourish, whether parts of your skin are covered by a thin layer or mermaid scales or you learn to move swift as the torrential currents, when you aren’t wearing armor, your AC equals 13 + your Dexterity modifier. You gain the ability to ignore any drawbacks caused by an underwater and stormy environment granting you resistance to cold damage.

Oceanborn

At 14th level, you can use your action to magically transform into a water elemental, you can use this feature once per long rest. Same limitations apply when in water elemental form as a druid in their elemental wildshape form. (Use the level 10 water elemental for Ac, Hp, Str, Dex, Con stats) This lasts for an hour, you can't concentrate on/cast spells while in this form.

Abyssal Undertow

Starting at 18th level, when a creature is held under any magical effect that affects their mind or has taken psychic damage this round you can use this feature to drag their mind into the dark abyss of the ocean and their fears.

Target becomes immobile for one turn(end of their turn), but immune to all damage and effects. They are mentally transported by your voice into an underwater landscape, where they are subject to all the horrors and nightmares you wish to inflict upon them. They can make a charisma saving throw equal to your spell save, if they succeed they take half damage and are just paralyzed for the remainder(remaining till the end of the caster’s following turn). At the end of your next turn, the target regains all of its senses, but begins coughing up salt water as if they had been drowning. During this time trapped in their mind they take 10d10 psychic damage and, unless they can breathe underwater, 5d10 cold damage as they attempt to recover from their horrific ordeal. Because of the horrors you have inflicted on them, as long as they remain within eyeshot, they have disadvantage on attacks against you for the next minute.

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