I have not cruised when pregnant...but we went to Hawaii when I was 20 weeks. I was sick my entire pregnancy. I used every trick in the book to try and combat the sickness. I had no problem going to Hawaii, but I did get a cold while I was there and of course couldn't take anything for it, so I was forced to stay inside in bed for one day. (Turns out it was the one day of the year it rained in Kihei, so I didn't miss out on too much!)
Also, before going to Hawaii, I told my OBGYN that I would be going to Hawaii and snorkeling and going on excursions. He said that I was allowed to take Dramamine. Check with your OBGYN for safety reasons. But aside from it making me tired, I believe it really helped on our snorkeling boat trip. especially since I get sea sick pertty easily. (on top of morning sickness.)
All of this said, I would not have been able to go before my 20th week because I was soooo very sick. But everyone is different.
Lastly, we are planning our next child around our next
Disney cruise. We are hoping to time it so that we are pregnant and cruising between the 18th and 24th week. We get pregnant pretty quickly, but we are aware that we run the risk of cruising before the 18th week. So, wish me luck!
And good luck to you with your decision!
P.S. Our last cruise, I saw the KEWLEST maternity bathing suit on a lady who was 20 weeks along. She got it at old navy and it was so kewl!!!!!