Hi H.,
I don't want to be an alarmist, and I do hope you'll take my two cents in the context of what others have to say. Based on a recent experience I had, I would have to say suggestion that you err on the side of caution and reconsider the cruise.
I was pregnant with triplets until a week ago (at my 17 1/2 week mark) when we lost one of the babies and nearly lost the other two. Thankfully, by no small miracle and with the help of a host of specialists and nurses, I am still carrying twins. Our only saving grace is that we had immediate access to top notch health care.
Up until the moment of my miscarriage my pregnancy had been incredibly smooth and we had no reason to worry, until suddenly, we did. I realize triplets are higher risk that twins, but still...
By all means, take a vacation; just consider staying on land and do a little research on the hospitals nearby so you'll know where to go if, God forbid, anything does happen. I recommend San Diego. I live there currently and am about to relocate to River Forest (once I'm cleared to fly). It's a beautiful place, and there are excellent hospitals here, including UCSD, which took care of me.