You probably should have asked under Disney.
No, Castaway Cay is NOT a big deal compared to the rest of your cruise. Every osland in the Caribbean has great beaches for you to visit.
Even on Castatway Cay you would be expected to pay extra for many things (snorkeling) so why nt just go to a good beach is Cozumel, etc.
Disney ships are more about the vessels, the shows and kids programs, not the destinations.
However, the Galveston cruises are selling at a good price, so it is worth going. Have fun.