Hi Krista,
Welcome to the Moms Panel! Each of our three children took their first Disney Cruise between the time they were 8-months to 1 year old. I can tell you that a lot of people thought we were crazy for taking them when they were so little - but we figured we earned a vacation and this was the easiest one to take with little ones. I loved having the quiet comfort of our stateroom only steps away should a meltdown ensue, and there was no need to lug a diaper bag because we could always quickly get back to our cabin.
There was always ample kid-friendly fresh food available, and laundry facilities helped cut down on the amount of luggage we had to bring.
Now even though your two-year old won't be eligible to participate in the
Oceaneers Club alone, you can bring him/her in there as often as you like to play and experience what is going on. You'll have to stay with her, but you can join in on the fun together.
When our kids were little they loved the evening dance parties, character greetings, movies and pool time. Castaway Cay features the newly opened "Spring a Leak" splashground which is a nice break from the beach.
As for sea sickness, this was a concern of mine as well. I spoke with my pediatrician and received very useful information. From my experience I can tell you that even on the cruises where I felt woozy, my kids never were affected.
Good luck with your decision - come back with all your planning questions!