Channelers that choose Water as their primary sphere will receive Flood as their prime specific weave. Flood is an attack weave which targets everything in the room that is not a part of your group. This includes not only mobs, but pets, mounts, and other players. However, Flood is currently disabled at this time.
Water prime channelers will have any of the remaining four spheres available to choose as their secondary sphere. The choice of secondary sphere strength, along with primary sphere strength, will determine the strength of weaves and stamina needed to cast.
For example, it is likely that a channeler choosing Water/Air will be able to cast a better Thirst weave for less stamina than a channeler choosing Air/Earth, but not as effective as on choosing Spirit/Water. Sphere strength and its effect on weave strength, duration, and stamina is complicated, and still being researched by the most experienced players.
Check the weaves page for the sphere strength of each weave.