I have sent a letter after every cruise letting them know what I liked and disliked (I've only had one dislike)
I've always gotten a letter in response. If they have done anything for the CM's that I talk about I don't know about it. But it's always nice to get a response.
I know the Cruise line always likes to know what they can improve on. So if there was something amiss during your cruise, please send them a letter.