Enjoy this vintage (1961) documentary about the Mermaids of Weeki Wachee, a tourist attraction in Florida that has existed from 1947 to present times. The underwater show features a hot spring full of performers in mermaid costumes, where viewers watch through a glass wall.
The mermaids breathe "from a hose supplying oxygen from an air compressor, rather than from a tank strapped onto the back. In the shows, the mermaids (and mermen — called princes) discreetly take mouthfuls of air from the slender breathing tubes while they perform. Even with the air tubes, though, it's clear that part of being a mermaid is being able to hold your breath for quite a while." (Weeki Wachee)
My favorite part of the documentary takes place at 6:05, when one of the performers effortlessly drinks from a bottle of soda under the water.