Deductions are allowed for the additional expenses you have had due to the rental. You get e.g. make a deduction for the fee to the brokerage firm and for the expense of the supplementary insurance that many insurance companies require you to take out when you rent out your boat.
Private cost of living does not entitle to a deduction
Other costs that you have for the boat such as. the mooring, bottom paint and launching you would have had in any case. Therefore, they are not counted as additional expenses but as private living costs that are not deductible.