Beginning at 1st level, you can call on your god parent to intervene on your behalf when your need is great.

Imploring your god parents aid requires you to use your action. Describe the assistance you seek, and roll percentile dice. If you roll a number equal to or lower than your player level, your deity intervenes. The DM chooses the nature of the intervention.

If your god parent intervenes, you can’t use this feature again for 7 days. Otherwise, you can use it again after you finish a long rest.

At 20th level, your call for intervention succeeds automatically, no roll required.

Having the blood of the gods coursing through your veins you have the innate ability to cast certain spells and cantrips. You also become proficient in a skill and your choice of a feat. Use your spell casting modifier if you have one for these spells otherwise use charisma. 

Eldrtich Blast

Cure Wounds at 1st level

Identify ritual cast

Message

Your mother a mortal and your father is the god of the seas. Poseidon’s blood is coursing through your veins. All of Poseidon’s children have particular traits. They all have bright glowing green eyes, strong swimming capabilities, able to breathe underwater indefinitely, and are able to communicate (telepathically) with any aquatic life and understand what they say (telepathically). 

Being the child of the sea god also gives you some innate magical abilities as you grow stronger. You currently gain:

Darkvision

proficiency in athletics

+1 to dexterity and strength 

proficiency in dexterity saving throws

As the child of Poseidon you gain the abilities to manipulate  water to do your bidding. You gain the following spells:

wall of water once per short rest 

watery sphere once per short rest

Control water can be cast at will

Demigods reach adulthood in their late teens and live to a max of 100 years. 

Humans vary widely in height and build, from barely 5 feet to well over 6 feet tall. Regardless of your position in that range, your size is Medium.

Your base walking speed is 30 feet.

Your base swim speed is 30 feet. 

You can speak, read, and write Common and one extra language of your choice. Humans typically learn the languages of other peoples they deal with, including obscure dialects. They are fond of sprinkling their speech with words borrowed from other tongues: Orc curses, Elvish musical expressions, Dwarvish military phrases, and so on.