A Water Baby is a commonly accepted term that is often used to describe those that are completely at home in bodies of water. This can include people who love to swim, surf, or dive. Often the term Water Baby can also used for those born in bodies of water. Lots of people who spend hours in the water often refer to themselves as a "Water Baby" despite their age, as it is commonly used to refer to adults as well as children. In surfing, the term water baby often refers to those that have the physical stamina to repeatedly paddle out and ride larger waves. over a prolonged period of time.
Its origins seemingly date back to the title of the Charles Kingsley novel, The Water-Babies, A Fairy Tale for a Land Baby (1863).