I have read that it's safe, but I wouldn't do it all the time. I think just a few times isn't a big deal. I was a HUGE spray tanner before I got pregnant, and I quit as soon as I found out... but I do plan to spray tan for my maternity pictures and baby shower. Most things are okay in moderation.
Having said that, I would try to minimize your exposure to the actual real sun once you're on the cruise. Make sure to use sunblock, and I would try to stay in the shade. You may burn more easily while pregnant, and you can also get melasma.. dark spots on your skin ... from the sun while pregnant.