We cruised several years ago when I was around 26 weeks pregnant. I had a note from my OBGYN saying I was cleared to travel, etc. They never even asked to see it when we checked in/embarked. What are they going to say, "Excuse me maam, but you look 27 weeks not 22!"
I'd just have a note from your doc to be on the safe side.
Now DD loves to say that she has been on the Mickey boat twice including once while in mommy's tummy. Happy cruising.