We just came back from a Disney cruise (7 night) - our first cruise and first vacation with our two kids. My oldest was 2 wks shy of his 3rd birthday, and my youngest was 7 mo. It was wonderful. My nearly-3 year old was in the pool all of the time, and when we were ashore, we took them to the beach for the morning, and then would come back to the ship for lunch & naps. Then after nap we would go back to the pool, then dress and get ready for dinner at 5:30, bedtime at 8 (a little later than when we are home). The ship was gorgeous, very clean, and everything was terrific.
Ethan (my 3 yr old) went to the kids' club 3 times, and Jack went to the nursery the same. It was hard to get time at the nursery, and I didn't want to leave him for very long because he is so little. They are very good there but the nursery charges $6/hr and has limited hrs & room. Jack was just the right age because he wasn't crawling yet, but was sitting, so he would sit on my beach chair with me while I watched my older son play in the pool with Daddy (or play in the ocean). Ethan was freaked about most of the characters except Minnie whom he loved. A few times we took the kids in their jammies to see part of the show at night - you don't need tickets, just walk in and sit in the back - and that was fun.
I think 18 mo though is not a good age for a cruise. NONE of the cruise lines can permit swim diapers in the pools because of Coast Guard regulations. Disney does have a small splash pool for little ones but it's too windy when you are at sea for most of the kids - they like to be really in the water to stay warm.
If you can wait until she is 3 there are a lot more things for her and you to enjoy together and apart. Plus the beach is way more fun at 3 than at 18 mo. Also the *extras* add up quickly - we didn't have any shore excursions, and had maybe a glass of wine or a cocktail each day, but our total *extras* was about $500 for the week when you included all of the tipping. And that doesn't include what we spent when we were ashore on food, cabs, and a few small souveniers.
I cannot wait to take my boys back to the Disney cruise - but I'm going to wait now until Jack is 3 and potty trained. Ethan will be 5 - it's only 2 yrs away! - and we'll have enough saved up to do it again. Wonderful place!