Try searching idealist.org for job and volunteer opportunities in non-profits all over the world. Using the keyword "water" or "agua" and selecting Spanish as a required language, I found the following (these were the most relevant in a long list of results):
Costa Rica - Building and Construction Projects: Build Homes and Communities (Playa Samara, Guanacaste, Costa Rica)
Intern with a Grassroots Environmental Organization in Peru (results also come up for the same organization's programs in Bolivia, Nicaragua, and Ecuador)
Construction Work/Housing Volunteer Abroad, Ecuador
Water and Health Consultants (La Ceiba, Atlantida, Honduras) - must have certain qualifications including "knowledge of water supply and water treatment technologies"
Permaculture/Sustainable Agriculture Volunteers (Sarchi, Costa Rica)
Environmental Volunteer Project in Ibiza, Spain!
Environmental research and reforestation in Costa Rica (San Gerardo de Rivas, Perez Zeledon, Costa Rica) - less about infrastructure and more about researching things, including water quality
An Additional Resource:
An Additional Resource:
The Committee on U.S.- Latin American Relations, whose site is hosted at Cornell, has a volunteer opportunities resource page, which may have useful leads for opportunities in Latin America, including (very near the bottom) this listing:
"Alternatives to the Peace Corps, The Institute for Food and Development Policy, 145 Ninth Street, San Francisco, CA 94103 (415) 864-8555, Directory of Third World and United States volunteer opportunities."
While it was published in 1994, the directory may still have some worthwhile leads. There is a nice abstract in the record from the ERIC database. You can use this link to find it in a library near you.
Subsequent addendum: the above work was apparently redone in 2003 by a different editor. You can use this link to find this edition in a library, or look for some of the other works on the same subject listed in WorldCat (a collection of library catalogs).