Currently, the Federal government offers a 30% tax credit to homes and businesses that install a solar hot water system. Most states offer solar water heating incentives as well.
The following link will direct you to the Database of State Incentives for Renewables & Efficiency (DSIRE). DSIRE is a comprehensive source of information on state, local, utility, and federal incentives that promote renewable energy and energy efficiency. http://www.dsireusa.org.