Our daughter is really strong willed. We found not potty training her at all worked beautifully. She actually trained herself shortly after she turned 3, and it was the easiest, most drama-free thing! All we did was to buy undies and let her know that we had them whenever she was ready to start using the potty all the time. No pressure. Then we bought a little potty and let her sort of "play" with it, sit on it whenever, etc. It helped that she was at a preschool that was super supportive of the casual potty-learning vs. potty-training method. She saw other kids using the potty at school and soon started wanting to try it out herself. It took her a few months -- largely because she transitioned to a new class shortly after she started, and it was too much for her to focus on/learn all at once, so pottying got backburnered. Once she was settled in her new class, she picked it back up on her own again, and was fully into undies two months later. We picked a long weekend and kept her at home so she had easy access to the potty. And we never looked back. She had no accidents that weekend, and only one at school (because she tried to hold it in while playing). Since nobody was reprimanding or lecturing or pressuring her about it, and because she's so independent, she had no trouble learning and adapting.
That said, I would personally not hassle with trying to train right before a cruise, if you are set on training. If you make progress that's likely going to get derailed by all the distractions and schedule shifts and the different environment of the cruise ship. By the time you get back home, you may have just wiped out all the progress you previously made. Instead, maybe show her the potty on the ship and casually let her know that if she wants to try it she can any time. And then let it go... focus on relaxing and enjoying your trip (for all of you), and doing what everyone is used to. Then if you really feel like you have to pressure her to learn to potty NOW, then do it once you get off the cruise. 3 weeks is not enough time to cause any significant changes in her personality or circumstances to make training any harder then vs. now.