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