We were on the eastern cruise in March and made our way to Coki beach on our own. It was myself, DW, DS (15), DD1(12), and DD2 (10). We found Coki beach fantastic for the children. Great snorkelling, helpful (not pushy) beach staff, and lots to see under the waves!
One of the largest attractions of Coki Beach, however, was the Coki Beach Dive Club. I am a certified scuba diver and had read up on them prior to my cruise. Needless to say I was not disappointed!
We ended up doing bubblemakers with my youngest daughter and the intro to scuba with my older. I also did a certified dive and saw all kinds of neat stuff! On the bubblemakers, the instructor brought along a bag of fish food and managed to get my girls surrounded by all kinds of colourful fish. They continue to talk about this even now! My older daughter has now assigned herslf the handle "scuba girl" and can't wait to get her scuba certification!
Sorry I don't know about sapphire beach - I hope this helps a bit about Coki beach!
Emjoy your cruise!
... Mike (aka Ottawacruiser)