Prof. Chris Harris from University of Cape Town, Department of Geological Sciences helped me for my δ18O analyses for granitic rocks. Maybe he could help you about your water samples.
This link http://web.sahra.arizona.edu/unesco/LABS_new_2008.pdf has a list of labs and what they can analyze. There is a lab at University of Arizona http://www.geo.arizona.edu/Environmental%20Isotope ; another at the US Geologic Survey https://isotopes.usgs.gov/lab/services.html ; and another at the University of Waterloo http://www.uweilab.ca/