But I agree with Chuck. I would at least get off and walk around on Castaway Cay. Just to see it!
Agreed.
Whaaaaaaa??????? But then I try to imagine why you would even consider not stepping off at Castaway Cay. You must not be a beachy person. That's my best guess. I can totally identify with people who feel this way because for YEARS I was as anti-beach as I was anti-cruise. Beach = hot, dirt everywhere (which makes me insane), and salty water that makes me feel sticky all over and my hair crunchy. Um, no thanks. LOL! Just as it took Mickey to get me to try a cruise (and fall in love), it took Mickey to get me to return to the beach (and fall in love). As a Disney fan I wasn't really interested in cruising per se. I wanted to see for myself the amazing, beautiful, incredibly detailed ship(s) Disney had created. It was @Disneyfalcon 's TRs that grabbed my attention. If I'd never seen her report and incredible pics from the preview of the Dream I would NEVER had stepped foot on a ship. Any ship. Then came Castaway Cay. This is Disney's island paradise. Most everything there they've touched and purposely themed in some way. The whole place is so packed with little details, morsels of unimaginable eye candy. As a Disney fan out to appreciate what Disney creates, I'd have been totally remiss if I didn't at least walk out on the island and look around. Thank goodness I gave it a little try! The theming that is everywhere is beyond anything I've ever seen in a park or resort. You can stand still, look at something, and the longer you look more bits of detail jump out at you. It's incredible. Honestly, it's an idealistic island that drips of Disney magic without being cheezy or overdone. It's like the perfect interpretation. Then, once you see that water, it will call to you. You'll need to put at least your toes in just to see the beautiful white sand and crystal water. No need to worry. There's fresh water showers and foot washes on the sides of the restroom buildings. Easy peasy to find. There's a few bars, as well, that you can pull up in the shade to have something cool to drink. Go grab an ice cream at the ice cream station and wander around. Look at the shops. Ride a bike. Go climb the observation tower. Ride the tram loop all the way out to Serenity Bay. You can still do a lot without ever leaving the paved surface if sand is an irritation to ya. 
Hey girl! Have to admit, I read your post and was like ---->
Like Tammy said, of the 2 stops on a short 3-night cruise I would stay onboard at Nassau before I stayed onboard at Castaway Cay. As a Disney fan, the experience of Castaway Cay holds far more value than Nassau ever could. That's the biggest thing that comes to mind. And, like Chuck said, there will be waaaaay less scheduled activities onboard on Castaway day than on Nassau day. So, to maximize time onboard and enjoy more of what the Dream has to offer in the way of activity offerings, Nassau as a stay-aboard day is your best bet.
Now, go google pics of Castaway Cay!
Honest, I wouldn't suggest you at least go look around a little if I didn't think it was worthwhile. I look at the beautiful pristine beach pics in magazines and think, "Yeah, if only it was like that (perfect)." Castaway truly does achieve something above and beyond any beach I've ever seen. And I'm accustomed to some of what are described as the most beautiful (Northwest Florida Gulf). Even if y'all just go take a ride on the tram and grab a drink at the bar out at Pelican Point, its something fun to do. I never thought I'd enjoy a beach. At Castaway Cay, I truly do. And no, I'm not a lay-on-the-beach person. At all!
Yep. After I got over the shock of such a thought it didn't take much for me to conclude you must not be beachy people. I can totally understand. I still don't consider myself a beach girl. I'm too picky for that.
BTW, if you'd like to see and touch an old 20k Nautilus sub, there's one sunk out in the snorkel lagoon. You can snorkel out to it. Lots of hidden treasure out there to see! Aaaaaand, that's what got me hooked on snorkeling. LOL!
The Flying Dutchman used in the filming of POTC had to be towed off. It was rotting. The Nautilus is still there. When you go out in the lagoon look for the buoys. They mark the points of interest. If you want to find something specific swim over to a lifeguard stand out in the lagoon and ask. They'll point the good buoys out to ya!
If I had to choose which port to stay onboard on, it would be Nassau.
We decided to stay on ship during Castaway Cay.
