The dives at Sha'ab Samadai aren't anything out of the ordinary, but snorkelling with the spinner dolphins which habitually spend the afternoon cruising around within the protective arc of the reef is extraordinary. I have done it several times over the past seven years and, while it has become very popular and there are too many people, it is still impressive to see 150 plus dolphins in the wild. The first year I did it there was our liveaboard – and that's it. We spent three hours in the water with over 200 dolphins and it was awe-inspiring.
As Christian said, Elphinstone rocks, and the arch is always interesting if you don't mind heading a bit deeper. But even in the shallows, the reef is alive with soft corals and reef fish, with plenty of pelagic action out in the blue.
St Johns has some nice walls and coral gardens as well, and these are little dived, so are usually pristine.
Mark