Hubby and I just returned from a cruise this morning (not a Disney one). Absolutely no water (even if sealed) is allowed on ship. This is standard practice for all cruise lines. Also, no potato chips (even if sealed), no gatorade (even if sealed) etc. and so on.
IF you need something specific, you MUST have a physicians note to go along with the item. And even if you, the custom officials can rightfully DENY the note and your request.
Hope this helps ~