Hydrokinesis

Hydrokinesis is the ability to manipulate and control water. This is a trait shared with water nymphs (Naiads, Oceanids, and Nereids), aquatic deities like Poseidon, Oceanus, and their offspring. Zeus also has hydrokinetic abilities, though he only controls the waters of the sky (primarily rain).

Uses of Hydrokinesis
As seen when performed by Percy Jackson and Poseidon, hydrokinesis is an extremely powerful ability:
 * Can have enhanced abilities when in water.
 * Live in water.
 * Control water to do virtually anything, as water is the most versatile element.
 * Have mastery over aquatic creatures.
 * Generate water.
 * Generate rain.

Percy Jackson

 * Percy, unknowingly, used hydrokinesis to pull Nancy Bobofit into a fountain.
 * He used it to defend himself (again unknowingly) from Clarisse La Rue by shooting water spouts out of the toilet, when she tried to dunk his head in the toilet.
 * When fighting with Ares, the god of war, Percy controlled the ocean waves to help him in the battle.
 * In The Lightning Thief (film), Percy used high density water to make a trident.
 * After Thalia Grace struck Percy with electricity, Percy angrily retaliated by summoning all the water out of the creek into an icy water typhoon to throw at her.
 * Percy used it to control the sea water in fossilized shells to clean out the horses' stables at Geryon's Triple G Ranch.
 * Percy used water from his own body to create a large blast of water at Mount Saint Helens. This is a very dangerous ability that left Percy very dehydrated and required him to rest for an extended period of time.
 * He willed himself to stay dry when walking through a river in the Labyrinth, as well as willing himself to get wet when he is about to get picked up by the coastguard in The Lightning Thief.
 * He used it to douse the fires in the battle at camp.
 * He was able to still the waters so Clarisse could drive her father's chariot across the water to Manhattan.
 * He controlled the waters in the River Lethe and willed himself to stay dry when submerged.
 * He can make bubbles which let other people breathe underwater as seen in The Sea of Monsters and The Last Olympian. Both the times, he created water to let Annabeth breathe.
 * He can walk on water.
 * He can breathe under water.
 * He partially used water when he created his own personal hurricane on two occasions, in The Last Olympian (against Hyperion) and in The Son of Neptune (against the legion of ghosts).
 * He used it to increase the water viscosity to a level where it was almost solid. He can use it to form shields or stand on water as if it were ground.
 * He can separate oxygen from water, making him able to breathe and create water bubbles when he is submerged.
 * He can create huge hands made of water from the Tiber River.
 * He blew up the Zhang family mansion's sprinkler system in The Son of Neptune.
 * He caused a humongous wave to break the Hubbard Glacier in The Son of Neptune.

Poseidon

 * In the war against Oceanus, Poseidon used all his power to keep the war under the sea from creating horrible disasters, like hurricanes, above the seas.Poseidon.jpg, god of the seas]]
 * He demonstrated an incredible amount of power over water when fighting Typhon along with the other Olympians.

Zeus

 * In Percy Jackson's Greek Gods, it is revealed that Zeus once brought about a global flood by causing tremendous torrents of water to pour down from the heavens all over the world for nine days and nights. As a result, the entire world was flooded (except for the highest mountains), and most of the human race was destroyed. Deucalion and Pyrrha were amoung the few that survived this great calamity.

Limitations
So far, the only major limitation of hydrokinesis is when used against certain beings (like other sea deities) who have a superior will over water and can wrest control from the weaker one. As well as being in a location where water is not abundant, or battle against a person whose ability can overcome or counteract water (e.g. Pyrokinesis). Also it appears that beings with hydrokinesis can control or create ice to a certain extent. Seen when Percy used the ice and snow around him to make an icy hurricane, as he does during the battle in Alaska in The Son of Neptune.