I would say that you have a good argument. If your reservation said that you would be provided with a roll in shower then I would think that would be, at best, denying you your reserved room type. At worst, it is denial of a medically necessary room requirement.

It does say that you need to talk to the hotel when it happens in order to invoke the Ultimate Reservation Guarantee. If you agreed to go to the other hotel then they may say that you basically OK'd the denial by leaving before calling them on the issue. If it is within 24hrs, and it is convenient, then I would go back to the hotel and discuss this with the manager. At the very least you should call them and discuss it.

If you get nowhere, I would contact Marriott directly. They can usually sort out grievances and they may throw in some points of their own.

Good luck and Happy Travels!