Hello and thank you for your question.
If you sell your boat for cash and buy another boat, then you are going to have to recognize the capital gains and IRC 1245 depreciation recapture as applicable. IRC 1245 depreciation recapture disallows capital gain treatment on the portion of a sale that pertains to prior years' depreciation deductions against ordinary income.
To avoid having to recognize gains all together, you may look to IRC 1031, which will generally allow you to exchange one boat for another boat in a tax-free exchange. See IRC 1031 here:
(a) Nonrecognition of gain or loss from exchanges solely in kind
(1) In general No gain or loss shall be recognized on the exchange of property held for productive use in a trade or business or for investment if such property is exchanged solely for property of like kind which is to be held either for productive use in a trade or business or for investment.....
I hope this is helpful to you. Thank you again!