If it's her first, you'll probably have many hours warning before the birth - it's called labour. It may not be comfortable, safe, wise or much fun to go into labour several hours away from the hospital, but 99 times out of a hundred, you wouldn't have to deliver the baby at sea. If it's her second or a later one, it will probably come quicker, but then she'll be a bit less frightened if labour starts. Can you sail somewhere local, so that you could get back quickly (say, 1 hour to somewhere an ambulence could pick her up) if things started happening?
Discuss the contingency plans with her, and if you are satisfied that she appreciates the dangers, then let her come. She's not ill - she's just pregnant - lots of women ride horses, run, sail, drive long distances... up to the time they go into labour.