Not just in the water it is in many drugs & vaccines too.
Thimerosal is a mercury-containing organic compound (an organomercurial). Since the 1930s, it has been widely used as a preservative in a number of biological and drug products, including many vaccines, to help prevent potentially life threatening contamination with harmful microbes.
But muh drug companies/agencies say it is perfectly safe coz it is not called mercury…duh.