water baby
noun
1A tiny childlike being who inhabits the water in, or in allusion to, Charles Kingsley's book The Water-babies (1863; serial publication 1862–3).
2A person who is likened to such a creature, as being happy or adept in or on the water; especially a person, especially a child, who loves swimming.
Origin
Mid 19th century; earliest use found in Charles Kingsley (1819–1875), novelist, Church of England clergyman, and controversialist.