If you decide to list it, it is a medical expense on the deductions and credits page. Because medical expenses are limited by the 7.5% rule, you may not get an actual benefit unless you have other significant medical expenses and you itemize your deductions.
Also be aware, when it it time to sell your home, you would normally increase your cost basis by the cost of permanent improvements such as a new furnace, new roof, or bathroom renovation. If you deduct the cost of the walk-in tub as an itemized medical expense, you can't also include it as a cost basis adjustment when you sell.
