The only "favorable tax law" in regard to the sale of inherited property is the "step up basis" (FMV at date of death) meaning that the inherited value is tax free. However, since you are selling to your sibling at a gain, you will have to pay Capital gains on the difference. You will be able to reduce those Capital Gains by major improvements (as you noted), but I do not believe you will be able to deduct to cost of the boat unless its sole purpose was to access the property which probably means it would have to be included in the sale.
Please look to this IRS link for further clarification:
https://www.irs.gov/faqs/interest-dividends-other-types-of-income/gifts-inheritances/gifts-inheritan...
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