Yes, you may add this amenity as a Rental Asset under Assets/Depreciation from the Rental Summary page for your rental property.
For purchases of $2,500 or more, add this as you would an appliance (equipment/furniture). TurboTax will ask you for the date you started using it and allow you to enter a business percentage less than 100%.
If the cost was under $2,500, you may expense the entire amount this year. First, follow the prompts under depreciation to take the Safe Harbor Election. Then go to Rental Expenses and add the hot tub on the page "Any Other Expenses?" with a custom description (see screenshot below - click to enlarge).
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