One of the reasons there is no uniformity in the courts is that the US Access Board has yet to come up with architectural guidelines for cruise ships. They had a committee make recommendations to them, and that committee disbanded several years ago. They have yet to publish any type of a report for public comment. Once they do this it will probably be about 2-3 years before any guidelines are adopted (lots of committees and public hearings in between and it could take even longer).
The ADA does not contain any access standards or architectural guidelines.
The ADA established that an entity (The Access Board) be created to establish architectural standards (the ADAAG). The ADA itself does not say for example that doorways must have a certain amount of clearance space or that toilets must be of a specific height in order to be considered "accessible"-- that is what the Access Board decided when it published the ADAAG. Right now we have no ADAAG for cruise ships.
So although Stevens v. Premiere Cruise Lines established that foreign flag ships that call on US ports are covered under Title III of the ADA, since we have no architectural standards it is pretty unenforceable. It has been interpreted to mean that people will not be denied boarding solely because of a disability.
Editor, Emerging Horizons
The only accessible travel magazine
***edited to remove commercial reference***