First thing, I think setting a date is a bad idea. I would get used to the fact that she will probably not be in the pool on this trip. Also, you might want to check the cruise line, because I know some of the pools are salt-water and REALLY cold so I doubt you would want her in that anyway. She might also sense your urgency and will probably rebel despite all your best efforts. Something to think about.
My daughter, who is almost 5 knew what to do at 18months. The first time I sat her there, she went! I was overjoyed...but it was short lived! She would wear a pull up all day, and then when I tried "Big Girl Panties" she would immediately go in her pants. Number 1 & #2!! By the time she was almost 3, she had mastered the #1! But it took a few months after she turned 3 to get the #2 down (I mean ZERO accidents by then). We offered 1 sugar-free gummy bear when she peed and 2 when she pooped. We also had a sticker chart. We had a little calendar thing and everytime she went, she got a sticker. Once she reached 7 stickers, she got a prize. We let her see the prize, but not touch it so she knew what she was working towards. One more thing, peer pressure helped a lot. My daughter is in school and by seeing the other kids do it, it really helped to get her motivated. She did want to be like the big kids and wear her big girl panties! She didn't want to be a baby anymore. Good luck and don't feel bad if she's not trained by 26months. In my opinion that is pretty young, even for a girl.