Sea sorcery

The power of water is the strength of flexibility,
resilience, and a relentless nature. Water parts to
allow a ship to sail over it or a diver to plunge
into it, but their passing leaves no mark. Water
flowing down a mountain reaches the sea. Itmight bend and turn across valleys and down
hillsides, but it slowly and steadily returns to the
waves. Those whose souls are touched by the
power of elemental water command a similar
power.
Your heritage ties to powerful creatures of the
sea, such as nereids, the lords of the merfolk, and
elemental powers. Like a river, you feel the call
of the ocean. The call is ever present in your
heart, and you are never completely at peace
until you are near the sea.

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. In addition, you ignore any of the drawbacks caused by a deep underwater environment, such as pressure or temperature. Lastly, you are granted Darkvision to a range 60 feet while underwater, falling to 30ft on land and nonexistent in dry/ arid environments. 

Curse of the sea

When you hit a creature with a cantrip’s attack
or when a creature fails a saving throw against
your cantrip, you can curse the target until the
end of your next turn or until you curse a
different creature with this feature.
Once per turn when you cast a spell, you can
trigger the curse if that spell deals cold or
lightning damage to the cursed target or forces it
to move. Doing so subjects the target to the
appropriate additional effect below, and then the
curse ends if the spell isn’t a cantrip (you choose
the 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.

Forced Movement

If the target is moved by your spell, the curse knocks the target prone.

Lighting Damage

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

Watery defense

You also gain the ability to defend yourself by momentarily assuming a watery form. As a reaction when you are hit by an attack and take bludgeoning, piercing, or slashing damage from it, you can reduce that damage by an amount equal to your sorcerer level plus your Charisma score, and then you can move up to 30 feet without provoking opportunity attacks. Once you use this special reaction, you can’t use it again until you finish a short or long rest.

Shifting Form

 

 Starting at 14th level, you gain the ability to enter a liquid state while moving.
When you move on your turn, you take only half damage from opportunity attacks, and you can move through any enemy’s space but can’t willingly end your move there. If you move through a creature's space in this way, you can choose to curse it as if you had used a bonus action, forcibly moved it or knocked it prone.
On your turn, you can move through any space that is at least 3 inches in diameter and do so without squeezing. When you stop moving, the regular squeezing rules apply if you’re in a space one size smaller than you. You can’t willingly stop in a space smaller than that, and if you’re forced to do so, you immediately flow to the nearest space that can fit you, back along the path of your movement.

One with the waves

Starting at 18th level, your being is altered by the power of the sea. You gain the following benefits:
• You no longer need to eat, drink, or breathe.

• A critical hit against you becomes a normal hit.
• You have resistance to bludgeoning, piercing, and slashing damage.
• Your Curse of the Sea no longer ends if you curse other creatures, and you can trigger more than one curse in a turn. Each curse lasts until triggered or until the end of your next turn as normal, and you can choose whether or in which way to trigger the curse for each creature separately.