We cruised when our daughter was 2 1/2, then again when our son was 10 months, and when he was 2 1/2. All three times we had a great time. There are plenty of excursions you can do with a child under 3, depending on which cruise you do. A lot of these are the sight-seeing type ones where you are mainly on a bus seeing things. They can be a little bit boring for the kids, but can also be a good way for you to see things and your child nap in the bus as you drive along
. Most of these type of excursions are free for children under 3, except if they involve a boat, where there is generally a cost.
My children both love cruising (my now 3yo son is constantly asking "can we go on the Mickey Ship now?"), and there is so much to do with them. Even when they are under 3. It just involves more of your time. E.g. there is Wake Up With Disney Junior, Open House in the Oceaneers Club (often with characters in the morning), the shows - both my kids at 2 1/2 LOVED the shows, getting photos taken - my son loved posing for photos
, playing in the splash zone if they're not potty trained, or in the pool if they are, both my kids loved spending time in some of the adult areas during the day when they were young - they were often totally empty so my kids would pretend they were on stage and 'perform', character meet and greets, dance parties / dancing to the bands in the lounge, etc. I'm sure there's tons more that aren't coming to me right now.
I think the nursery is very reasonably priced, and we did book it in advance to go to Palo, and also if we had had a long excursion day planned where we weren't sure whether our son would get his full nap or any nap at all, we booked the nursery for about an hour after dinner started. So took him to dinner, let him have his, then put him in the nursery so he wouldn't get grumpy waiting for us to finish eating.
Hope that helps.