Something to consider (and this is the "horror story" part so don't read on if you don't want to hear it).....
I think it's not just about the "viability" of the baby if you go, but also the safety of the mom. Without giving every detail, I will just say that I delivered and lost my twins at 19.5 wks, plus I spent 3 days in the hospital for losing so much blood. Now I realize I am in a small percentage, but I never thought I'd be in it. I had a picture perfect pregnancy up until the day I went into the hospital so there was no reason to not sail (In fact, I sailed at 12 wks pregnant with them!). Plus this was pregnancy #3- I already have two healthy DDs so I really had no worries.
SO- yes, I would and DID sail up until the 24 wks limit. But now knowing that even before 24 weeks could end up like this, I would definately think twice now.
If you do decide to sail, I would check with your doc and know all the risks and possibly even spend time online checking out the medical facitilies at some of the ports.
No matter what, I wish you the best with your pregnancy! And if you do put it off, I can also say that both my DDs sailed as young infants and that was fun (not as relaxing as it would be BEFORE you have them, but still good!)