You can breathe air and water.

Aquatic beasts have an extraordinary affinity with your people. You can communicate simple ideas with beasts that can breathe water. They can understand the meaning of your words, though you have no special ability to understand them in return.

Your Strength, Constitution, and Charisma scores each increase by 1.

Sea serpents reach maturity around age 15 and can live up to 200 years.

Sea serpents tend toward lawful good. As guardians of the darkest reaches of the sea, their culture pushes them toward order and benevolence.

Sea serpents are slightly shorter than humans, averaging about 5 feet tall. Your size is Medium.

Your base walking and swimming speed is 30 feet.

You have draconic ancestry that manifests in a cold breath weapon and resistance to cold damage.

When you use your breath weapon, each creature in the area of the exhalation must make a saving throw, the type of which is determined by your draconic ancestry. The DC for this saving throw equals 8 + your Constitution modifier + your proficiency bonus. A creature takes 2d6 cold damage on a failed save, and half as much damage on a successful one. The damage increases to 3d6 at 6th level, 4d6 at 11th level, and 5d6 at 16th level. After you use your breath weapon, you can’t use it again until you complete a short or long rest.

Adapted to even the most extreme ocean depths, you have resistance to cold damage, and you ignore any of the drawbacks caused by a deep, underwater environment.

You can speak, read, and write Common and Primordial.