I have stayed at any number of Marriott properties over the years and I do not recall ever having seen a walk in tub. However, I believe this is a question for Marriott Corporate to answer. I suggest that you call 1800.Marriott and inquire about your special need.
good luck, and let us know what you learn.
We took a deeper look into this for you. While this is a great question, unfortunately there is no way for the team to search for a hotel based on this amenity request. The other posts on this thread are pretty spot-on! We too recommend that you give Marriott a call at 800-228-9290. The agents are happy to help you narrow down the cities and brands which may fit your needs best.
Hope this helps!