If you're building a Sea Child from scratch, rather than using a Subrace or Variant, then your Wisdom increases by 2, and your Charisma by 1.

You can breathe both air and water.

Wile underwater, you can perceive your surroundings without relying on sight within a 15-foot radius.

Your eyes are built to be able to see in the impermeable darkness of the deep. You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim. You can't discern colors in darkness, only shades of gray.

When underwater, you can see as clearly as on land and suffer no penalties or disadvantages on your movement or attacks.

Along with whatever languages you would know based on your race of birth, you now know how to innately speak, read, and write in Aquan. You are also able to understand the other dialects of Primordial, and can roughly communicate in the those dialects as well.

You are resistant to both acid and cold damage.

While submerged underwater, you can use an action to change your body from your waist downwards into that of an aquatic tail. It is your choice as to whatever clothing items you were wearing morph with you or not. In this "Merform," your AC goes up by 1, thanks to the sprinkling of tough scales that have appeared over your body. Your swimming speed also goes up by 10.

 If you are out of the water for more than one minute, your tail morphs back without the use of an action.

Though not all sea beings are known for their voices, Children of the Sea are quite lucky in this aspect. Because of this, you can use you voices as a spellcasting focus should you have need of one. As an action, you can also use your singing to affect a creature that can hear and understand you as the Friends spell. The target isn't immediately hostile when the effect ends but they can still be angry with you. You can use this ability once per short rest.  

Because so many kinds of entities can make someone into a Child of the Sea, Children can vary in just as many ways. Thus, you can chose one out of six racial traits for your Sea Child: Envoy of the Deep, Grasp of the Lurker, Luminescence, Mask of the Depths, Quagmire Cloak, or Sincere Believer.

You will always carry a spark of the magic that made you into a Sea Child. This magic manifests in different ways that you may choose from;  Call of the Aquatect,  Child of the Deep,  or Semi-Elemental Control.