I'm an Obstetrician and here a few thoughts:
-Health-wise there is no problem with cruising at 18 weeks. I would recommend getting beyond around 13 weeks due to you being very prone to nausea prior to then and you may find that motion sickness is a problem even when it isn't when you're not pregnant.
-There are very few restrictions on types of massages you can have during pregnancy.
DCL offers a pregnancy massage as well if I'm not mistaken. Be careful of a lot people telling you "you can't do that during pregnancy." They are usually basing it upon things like "my friend told me" or "I read on the internet." Just make an appointment with your OB the week prior to your leaving and talk about what you can and can't do.
-Avoid the large salt-water fish for meals like shark steak, swordfish, tuna steaks, etc. These fish are much older and larger when they are caught and have fairly high mercury levels compared to others. For the same reason, I would avoid filter-feeding things like oysters.
-Most cruise lines make the cut-off for going at 24 weeks. Any OB should have a form note stating what your due date is and how many weeks you will be.
-Drink, drink, drink. Your fluid requirements go up a great deal when you are pregnancy and you have a very low threshold for becoming dehydrated. Usually one of the first symptoms of dehydration is contractions. My rule of thumb for expectant patients is if you are not having to pee every 90 minutes while outside, you're not drinking enough. Try to make the majority of your fluid intake non-caffeinated and non-carbonated. Water is the best, but juices (as long as you're not diabetic) and sports drinks are good, too.
-Find shade. Avoid getting overly hot. Not only do you lose more fluids then, but the body's natural reaction is to shunt blood flow to the skin to dissapate (sp?) heat. This means you are shunting blood flow away from internal organs such as the uterus. This is also why hot tubbing is not a good idea. Adequate fluid intake will help with this as well.
-Enjoy each other. With the exception of a few conditions that your OB can tell you about, sex is very safe in pregnancy.
-Put others to work. Your center of gravity changes as your pregnancy progresses and you are more prone to falls especially when carrying big loads. Let someone else carry the big pieces of luggage.
Hope these tips help and have a great time and congratulations on the baby.