The IRS just recently published Form 5695 for 2018. This is the IRS form that you would need to complete to take advantage of the 30% Federal Tax Credit if you placed your solar system into service in 2018 (or earlier).
Who Can Take the Credit?
"You may be able to take the credit if you made energy saving improvements to your home located in the United States in 2018. A home is where you lived in 2018 and can include a house, houseboat, mobile home, cooperative apartment, condominium...The home doesn't have to be your main home."
So long as your boat (classed as your first/second home) has a head, galley & bunk and is registered in the USA then you may be eligible to use the Tax Credit.
Please consult with your tax advisor or accountant.
Don't miss the boat!
We have a whole page on the website about the Solar Tax Credits. The credit is currently at 30% until December 31st 2019. It will then be reduced to 26% in 2020 and 22% in 2021. After that the Tax Credit is no more and you missed the boat!
Download the instructions for IRS Form 5695 here www.irs.gov/pub/irs-pdf/i5695.pdf
Download IRS Form 5695 here www.irs.gov/pub/irs-pdf/f5695.pdf