Water acts may ebb and flow, but since 1950 the polyester-clad mermaids at San Marcos’ Aquarena Springs have barely had time to keep their heads above water. Their subaquatic dances are a tribute to the popularity of such swimming celebrities as Esther Williams and Johnny Weissmuller, a testament to lung power, and proof of the appeal of hokiness. Some of the shows follow fads: The myth of the sunken city of Atlantis was a hit in the seventies. The current act features an Indian love story. But the Mermaid Picnic has remained the same. For 42 years, a swimmer, dressed to the gills in blue satin fish scales and a gold top, sits on a subterranean lily pad, unpacks her basket, nib-bles on a pickle, gulps a soft drink, and then balances the bottle on her nose. Will this old standby go the way of Atlantis? Don’t hold your breath. Says David Edwards, the attractions supervisor at the springs: “Ralph the Swimming Pig and the Mermaid Picnic will never change.”