The Royal Suites have both a tub and a walk in shower. They also have cabins with showers that have no lip at all... so a wheelchair can be wheeled right in. We got one of those cabins once, even tho we didn't need it. The shower was huge!
When you call your agent let him/her know that you require a cabin with a shower that you can walk into. Make sure they know it's a requirement, not a request. You shouldn't have any problem unless they are already booked by people that also require it.