The US Federal Government offers a solar energy tax credit, applicable to your second home. If you have a head and galley onboard and your boat is docked in the United States, your boat qualifies as a second home. This tax credit,extended to December 31, 2019, is a 30% credit on qualified expenditures to purchase and install solar panels placed in service after 2008. This credit can be retroactive – as long as the solar system was placed into service after January 1, 2006.
The Department of Energy website gives a simplified description of the energy tax credit: http://energy.gov/savings/residential-renewable-energy-tax-credit. The actual IRS form is easy to fill out: http://www.irs.gov/pub/irs-pdf/f5695.pdf. It may also worth investigating your state government solar tax credit.