Bought a boat for what?   Is he in a business that uses a boat?  Is it a second home for him perosnally?

bought because he is a fisherman LLC on a sch C 

The IRS rules on depreciation allow a fishing business to depreciate boats over a span of seven years; a 10-year depreciation applies to fish tenders and fish processing boats.

He'll also have the interest expense on that loan to deduct.

@Just-Lisa-Now- So how do you do that? 

Do what?

You set up the boat on Sch C, Asset entry worksheet, use Code G (I think?) for 7 year equipment.

Interest paid on the loan in 2022 would go on Line 16. 

If this is a commercial lender he should get a statement that shows Interest paid each year, if its a private seller you may want to create an amortization schedule so you know what the interest is each year.