Wavesoul Sorcerer

Your innate magic comes from connection to the Elemental Plane of Water.  You might be a mortal descendant of a marid or perhaps a distant ancestor made a pact with the primordial forces of that plane.

Elemental Ancestry

At 1st level, you learn the Shape Water cantrip. This counts as sorcerer spell for you, and does not count against your number of sorcerer spells known.

You can speak, read, and write Primordial. Primordial is a guttural language, filled with harsh syllables and hard consonants.

Fluid Structure

Your body is extremely flexible. You gain advantage on all ability checks and saving throws to avoid or escape being restrained or grappled.

Additionally, you have a swim speed of 30 feet and you can breathe both air and water.

Resilience of Water

Beginning at 6th level, you can reconstruct your physical form as an act of will. As a bonus action, you may spend up to 4 sorcery points. For each point spent this way, you may heal yourself for 1d6 + your Charisma bonus.

You also have resistance to poison and acid damage.

Elemental Form

At 14th level, you can use your action to magically transform into an water elemental. You can use this feature once. At 18th level, you gain an additional use. You regain expended uses when you finish a short or long rest.

Unending Flow

At 18th level, your movement is unaffected by difficult terrain, and spells and other magical effects can neither reduce your speed nor cause your to be paralyzed or restrained. You may spend 5 feet of movement to automatically escape from nonmagical restraints, such as manacles or a creature that has you grappled.

Being underwater imposes no penalties on your movement or attacks, and while completely submerged you have blindsight with a range of 10 feet. Within that range, you can effectively see anything that isn’t behind total cover, even if you’re blinded or in darkness. Moreover, you can see an invisible creature, unless the creature successfully hides from you.

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. 

Additionally, 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.