A waternymph, or a rivernymph maybe? I love the stories about the Rusalka, or even better (more tragic), the Rusalki.
"Rusalka, a quiet, undead being. The soul of a drowned woman who had died in or near a river or a lake and comes back to haunt that waterway. This undead rusalka is not invariably malevolent, and will be allowed to die in peace if her death is avenged"
See detail of the nymph here: [link]