I have to admit, I am always surprised by the advice to run, run, run to get a "good spot". We didn't dilly-dally along but we kept our island time pace and did just fine. We have been to
Castaway Cay three times and have gotten a great site each time. Soooo many people get off the ship and stake a claim at the first patch of beach they come to. Where you pick up snorkeling gear is just packed! As you walk further down the beach it becomes more and more sparsely populated.
IMO the best place is the area by the water volleyball net. There is a bathroom closeby as well as the great bar over the water (where you can get sodas in a can). There are also some wonderful hammocks a little further down (now, those might be worth hustling for). Someone correct me if I'm wrong but just on the other side of the bathroom I mentioned is a beach area that had maybe ten people max each time we were there and I don't think it is the teen beach.
As someone else said, the adult beach is a MUST. We missed it our first trip there but were thrilled when, on the same cruise, we got to hit CC again (our cruise had a change of itenerary because of bad weather and we were fortunate enough to do CC twice that trip!!!) and spent the whole time there after dropping the kids off to play.
Have to agree about the hippo remark, too! When you see how common that it, you'll get a good laugh!
Wherever you end up on the beach just remember everything is "island time" so sit back, sip a cocktail and HAVE FUN!!!