Check out these grants that could be used for rain gardens, gardens or other projects involving fresh water:
GreenWorks! Grants (deadline end of September)
From Project Learning Tree (with the US Forest Service)
GreenWorks! is the service-learning component of PLT that provides grants to PLT trained educators for students to implement environmental improvement projects. By blending comunity service with the academic curriculum, students are "learning by doing."
Environmental Education (EE) Grants (deadlines in late winter)
From the US Environmental Protection Agency
EPA seeks grant proposals from eligible applicants to support environmental education projects that promote environmental awareness and stewardship and help provide people with the skills to take responsible actions to protect the environment. This grant program provides financial support for projects that design, demonstrate, and/or disseminate environmental education practices, methods, or techniques.
Captain Planet Foundation (deadlines twice a year)
Grants are made for activities that conform to the mission of the Captain Planet Foundation and MUST have all three of the following to be considered for funding: be project-based; performed by youth; and have real environmental outcomes. Grants from the Captain Planet Foundation are intended to: provide hands-on environmental stewardship opportunities for youth; serve as a catalyst to getting environment-based education in schools; and inspire youth and communities to participate in community service through environmental stewardship activities.