WordNet sense 1 (submerged aquatic plant having narrow leaves and small flowers; of fresh or brackish water):

naiad

WordNet sense 2 (a water lily having large leaves and showy fragrant flowers that float on the water; of temperate and tropical regions):

fragrant water lily, Nymphaea odorata, pond lily

WordNet sense 3 (a fairy that inhabits water):

water sprite, water spirit