"There must be something in the water” is a jocular statement, guessing that the water is responsible for unusual conditions in a locality. “There must be something in the water at St. Matthews that make people like to scrap” was cited in print in 1909. “There must be something in the water,” referring to a town with a high pregnancy rate, was cited in 1910.
7 October 1909, The Times and Democrat (Orangeburg, SC), “Local News items,"pg. 4, col. 7:
There must be something in the water at St. Matthews that make people like to scrap.
5 March 1910, Oakland (CA) Tribune, “Stork’s Home Located,” pg. 7, col. 4:
“If you know of any childless households that long to change that condition, send them up here. There must be something in the water. I know several women who have visited up here from the city, and who regarded themselves as absolutely stork-proof, but found out their mistake—as I did.—The Wasp.
18 May 1938, The Bakersfield Californian (Bakersfield, CA), “Pipefuls” by Jim Day, pg. 12, cols. 4-5:
There must be something in the water or food down here that makes Bakersfield people win so consistently.
OCLC WorldCat record
There’s Something in the Water or the Air! Words by Hal Richardson. S. A. T. B..
Author: John Klein
Publisher: New York : Boosey & Hawkes, 
Edition/Format: Musical score
9 January 1964, Omaha (NE) World-Herald, “Numbers Game” (Miami Herald), pg. 22, col. 2:
If you have been around a baseball training camp, you get the impression there must be something in the water in Louisiana that producers big leaguers.
30 August 1965, San Diego (CA) Union, “Lincoln Gets Rough Handling” by Jack Murphy, pg. A-8, col. 1:
It must be something in the water. First, San Francisco pitcher Juan Marichal hones his bat and the skull of John Roseboro. Now Al Davis, Oakland’s resident football genius, is suspect of dispatching a couple of padded goons to inflict harm on the person of Keith Lincoln.
IMDb (Internet Movie Database)
The Stepford Wives (1975)
Bobbie Markowe: I think there’s something in the water that turns us into house fraus.
Carrie Underwood - Something in the Water
Published on Nov 6, 2014
Music video by Carrie Underwood performing Something in the Water. (C) 2014 19 Recordings Limited, under exclusive license to Sony Music Nashville, under exclusive license to Sony Music Entertainment