Essentially sentient water, The Waterhive are certainly one of the strangest races The Doctor has ever encountered, being a hive consciousness controlled by a queen that operated in water molecules.
In their natural state, The Waterhive resembled large one-eyed eels that could grow even larger and manifest fins and enlarged mandibles to intimidate their prey. Capable of controlling moisture, The Waterhive could draw in water from nearby humans, using water to create three-dimensional bodies for themselves that could convincingly mimic human beings, further aided by their telepathy as they absorbed knowledge from their deceased victims. They could also re-animate and control human beings who drowned, either through their actions or just in their vicinity, using their knowledge of anti-cellularisatedition to alter the victims’ bodies, giving them gills and fish-like eyes; even when they simply preserved the bodies, the hosts’ eyes would eventually become ‘pearly’, requiring them to wear glasses. The most disturbing part of their lifecycle was that they laid their eggs in human bodies, providing their young with a source of nutrients when they hatched, luring victims to them to serve as future hosts by manipulating the emotions of drowned humans and projecting illusions of them to the victims’ friends and relatives.
During a visit to Earth, the Tenth Doctor and Rose learned about The Waterhive when Rose’s old friend Keisha revealed that she was seeing her recently-drowned brother Jay. Investigating the ‘ghost’, The Doctor discovered that twenty people had gone missing in the area around the Thames, while Jay’s ship, the Ascendant, had been brought up the Thames to a building called Stanchedition House, The Doctor infiltrating the building while Mickey and Rose questioned various relatives of those lost on the ship, all gathered around the Thames and trying to jump in to reach their loved ones before ‘the feast’. Although The Doctor’s infiltration was exposed, he was still able to gain access to the area where the Ascendant’s remains were stored, noting that it was cut up in a manner that should be impossible for human technology.
Noting the anomalous presence of salt water around the wreck when it had been taken via the Thames, The Doctor fled down a tunnel, where he witnessed another manifestation of Jay being dragged away by other drowning victims in older clothes. While Rose discovered that nobody jumping into the river had ever been found, The Doctor determined that the illusions witnessed were trying to deliberately drown people close to those whose appearances they had adopted for some other purpose, with people suffering from fits in the area where the apparitions appeared as they were drawing on the water in the bodies of those around them to sustain themselves.
Having determined that there were unusual elements in the water where the Ascendant had been, The Doctor deduced that their enemy were trying to drive people to drown themselves. Rose and Keisha’s contact with the apparition of Jay had already started the conversion process in them, although Keisha’s was more advanced due to Rose’s lesser exposure, and the creatures could only transmit to people acquainted with those they had captured; as an example, Jay had only been visible to Rose and Keisha despite Mickey being in the room as well because Mickey and Jay had never met.
Searching records for information on the Ascendant, Mickey and Keisha learned that Rear Admiral Crayshaw, the man in charge of the Ascendant wreckage, had been around for over two and a half centuries, allowing The Doctor to determine that the Waterhive had infiltrated the Navy using Crayshaw as their inside man. Although Rose was captured when she tried to follow a scientist who had been captured by The Waterhive’s ‘agents’, this allowed her to learn that their enemy was the ‘waterhive’, later managing to resist The Waterhive’s influence when she learned that they intended to use the living drowned as human incubators for their ‘eggs’. With Rose’s resistance helping The Doctor realise that it was possible to fight against The Waterhive, he pretended to offer The Waterhive a deal where he would take them to another planet in the TARDIS, knowing that they would try to capture him and take the ship by force. As he was linked to The Waterhive’s queen, The Doctor used his own mental powers and the inhibitive powers of a chemical transmitter released into the water to attack The Waterhive’s chemical communication signatures, allowing him to disrupt it for good as it couldn’t process the signals it was receiving. The same attack caused The Waterhive’s eggs to collapse and restore its genetically-damaged victims to normal, essentially restoring all of the Waterhive’s victims to life and humanity while The Waterhive itself fell apart.