I took a 17 day cruise in January for work, believe it or not. My wife and I led a student group on the trip and we did a service project in the AM at every port of call that we went to. Certainly, it is not an ideal situation, but I was able to get in some good training. A few tips that I came up with:
Your definition of a lap pool and theirs are likely very different. Thankfully, I brought a swim tether with me and tied it to the ladder. (
http://www.swimtether.com/) I did a lot of 30, 45 and 60 second intervals on it with short rest. On our port days, I got an open water swim in the afternoons after our project. The tether will pull your hips down and force you to go to your legs quite a bit. People will look at you like you are crazy, but no worries! Also, try to swim into the waves. The boat will rock side to side sometimes and forward to back others.
The Gym typically does not have great ventilation. Bring a lot of water. I did a couple 5-6 hour rides on the spin bikes they had in there and it was pretty hot and stuffy. On the plus side, I got to watch some whales who were playing by our boat for a few hours during one of the rides! Pretty cool. Bring your HRM with you since you probably won't have power. Pretty easy to get the hr up and keep it there for an extended period of time. You will sweat...a lot. And it probably won't evaporate much. Throw a towel behind the bike and in front of the bike. First day, I didn't do this and someone slipped in my sweat...at the other end of the gym! The rocking boat moved the puddle of sweat and I didn't even realize it.
The treadmills are usually pretty good in the gym. However, watch the weather. A few days the water was really choppy and you really couldn't run at all on a moving treadmill. The captain will give a weather report every morning, pay attention to the winds and plan your runs for days that have a little less wind. Again, I tried to get my longer runs in on dry land whenever possible.
Your running and/or biking shoes will likely be soaked every day. Take the inserts out and put some news paper in them to draw out the moisture and so they don't stink...so bad. I brought a pair I could just leave in the garbage on the ship at the end.
Be flexible with your workout plan, get in what you can, relax some, and of course, enjoy the trip!
Hope this helps a bit.