On our first Princess cruise, one of our tablemates was a strict vegetarian - no milk, no eggs, etc. I don't know if it was indicated in her documents, but on the very first night, she told the head waiter that she was a vegetarian. Every evening, he would bring her the next days menu and she would let him know what she wanted. He also gave her a few choices of items that were not on the menu, but I can't remember what they were.
I know she enjoyed her dinners every night and we enjoyed having her as a tablemate. She was 87 years old and came from Sussex, UK. She was traveling with her 82 year old friend. The two ladies were a riot.