Nymphs are Silvers, who can manipulate water. Like the other Silvers who can only manipulate their own element, nymphs require a nearby source of water to control. However, they have been known to leech water from the air.
"Nymphs are elementals, manipulators of moisture, allowing them to control water in varying measures of strength, dependent on skill and natural ability. The royal family of the Lakelands, the extensive line of Cygnet, are extremely powerful nymphs, well known for maintaining direct lines of descent within their own family tree. Kings and queens of the Lakelands have been known to raise the shores of their borders in defense, drowning entire fleets. In Norta, House Osanos is also an important nymph family, though they are eclipsed by more powerful houses like Calore, Merandus, and Samos. - On Silverblooded Abilities by Victoria Aveyard