Short answers, pulled from the story.
The Greek word naiad derives from the verb náo meaning to flow and connects to naía which describes a body of flowing water. This etymology defines the spirit's domain as fresh water sources like fountains and springs rather than still waters or entire rivers.
Naiads often ended interactions with humans through loss or transformation such as when they pulled Hylas into the water forever due to his beauty. They also punished betrayal with permanent blindness as seen when Nomia blinded Daphnis for his infidelity or merged with Hermaphroditus against his will.
Ancient speakers worshipped these female spirits at pre-Mycenaean sites like Lerna in Argolis where lagoon-lakes held such spirits. Local communities dedicated childish locks at the spring of Lerna during coming-of-age ceremonies and performed ritual cleansing and animal sacrifices there.
Yes the water nymph associated with particular springs survived across Europe without direct Greek connection through Celtic wells rededicated to Saints. Medieval legends like Melusine carried forward these ancient beliefs while communities maintained rituals honoring local springs throughout the Middle Ages.
Modern uses include Naiad Lake in Antarctica and the innermost moon of Neptune which carries the title Naiad. St. Charles Avenue in New Orleans was formerly known as Nyades Street running parallel to Dryades Street which honors tree spirits.