I can post items that would be deductible were it a rental home. You can see if any apply to you.
Sales Expenses for selling your property include:
Sales commissions
Advertising Expenses
Legal Fees
Broker Fees
Transfer taxes
Cost Basis is the rental's purchase price plus buying costs (fees you paid in connection with the purchase such as legal fees, abstract fees, survey charges, owner's title, etc.) plus improvements, minus depreciation.
Adjusted basis is the rental's purchase price plus buying costs plus improvements plus sales expenses, minus prior-year depreciation.
A boat slip would not fall under any regular investment classification. You may enter it as "Everything Else". Note that any gain or loss would be reported as Long-Term or Short-Term Capital Gain/Loss and taxed as such.
Can expenses such as Property Tax, Loan interest, insurance, or slip rental be included in the cost basis when determining capital gains from the sale of the boat? I am selling a boat for more than I paid for it, and the capital gains tax may be very high.
No. The expenses you describe are all ordinary and necessary personal expenses for owning and using/operating the boat but do not improve or increase the value of the boat itself. Only expenses that are capital improvements to the boat itself are considered part of the cost basis.
You can include any expenses paid to make and close the sale of the boat.