The user can manipulate water in every form possible, the basic three being liquid, solid and gas, as well as transform it from one phase to another. More powerful users can convert water into the fourth phase, plasma.
The user can also manipulate every property of water, such as viscosity and density of water, changing liquid into thick and adhesive or thin as oil, or condensing vapor particles and liquids enough to be tangible to the point where they are solid and hardened.
Because blood cells are 55% blood plasma which in turn is up to 95% water, advanced users can control blood via its water content. This allows them to control the target, and even freeze or dehydrate their blood. This would take precision as blood plasma contains other components.
Hama (Avatar: The Last Airbender) is one of the most powerful waterbenders, and can control water in all its forms, such as extracting water from plants and thin air and control body fluids in the human body.
Katara (Avatar: The Last Airbender) is a powerful waterbending master, and can control water in all three phases, such as extracting water from plants and thin air and can even control bodily fluids in the human body.
Yukihina (Code:Breaker) using his Shape of Water ability to control his own blood by vaporizing it, and then injecting into Heike and freezing it from inside Heike's body.
Tokiya Mikagami (Flame of Recca) wielding Ensui, a sword that gathers water molecules and can control it in any of its three phases, making it a very versatile weapon.
Genbu (Magic User Kurohime) is the Water Spirit King, and can control water of all three phases, allowing anyone who subjugated him to control the water element magic to the highest level.
Saiyuki (Magic User Kurohime) gains Genbu's power by subjugating him, and she can freely shift between liquid, ice and vapor forms as a result.
Yuki-Onna (Magic User Kurohime) is a Goddess who controls snow and ice, freezing water into such, as well as shifting opposing ice-based attacks into harmless water.
Ororo Munroe/Storm (Marvel Comics) can control the weather at tremendous levels, and can therefore manipulate water content within the air as precipitation, humidity and moisture.
Toma Zaslon (Marvel Comics) controls H2O in all of its forms.
Bridget O'Hare/Sea Witch (Marvel Comics) had the power to control water in any state (whether solid, liquid, gas).
Haku (Naruto) possesses the Ice Release, which allows him to reconstitute liquid water into ice, and dropping the external temperature so much that it creates a frosty mist and snow.
Chaos (Sonic the Hedgehog) is composed of liquid chaos energy, to which it can manipulate, and shift its form to any state of that it desires.
Europa (Valkyrie Crusade) as the goddess of water, can manipulate water in any shape and form.
Omi (Xiaolin Showdown) is the Dragon of Water, and can manipulate water in any shape and form, such as liquid, ice and snow, and give them shape.
Isaac McDougall (Fullmetal Alchemist) is a water alchemist who can transmute it and change its state form ...