Would she go on an unwanted, expensive (still) week+ long trip spending a week on a ship with a seasick husband? I'm guessing she would beg off.
It sounds like you're her friend as long as you do what she wants...kind of a like a 2nd grader. It's time for her to leave the schoolyard tactics behind and grow up. Tell her one more time that you do support her, but you just can't go and you don't wish to discuss this anymore. Either she'll accept and forgive you for what she sees as a slight or she won't. If she doesn't do you really need a friend like that?