I give Arthur C. Clarke credit for introducing me, many decades back, to SF characters who were truly three-dimensional. He brings that same sympathy to bear in Dolphin Island. Unfortunately, the rest of the story doesn't hold up as well, even for a young adult story.
While Clarke clearly has a strong affinity for dolphins (who doesn't?), it's curiously divided. He acknowledges that dolphins are sentient and intelligent, and that orcas are their close cousins. Yet he sees little wrong with capturing an orca, winning its trust, and sticking control electrodes in its brain. Our hero feels a momentary uncertainty, and that's about it for morality.
The strength of the book is in its loving description of the Great Barrier Reef and its ecology. While some aspects will give a modern reader pause (hey, a clutch of turtle eggs - let's dig them up and eat them!), Clarke is generally spellbound by the free-diving possibilities and the beauty of the ecology, and does a very nice job of presenting them. In fact, one could almost say that the story is more an excuse for writing about the Reef than vice versa.
The story itself is functional YA - nothing extraordinary. Clarke manages his usual success of making the key characters credible and reasonably well-rounded, but only the boys in the forefront really develop. The story simply doesn't match the mastery of Clarke's more substantial efforts.
All in all, a decent but slight Clarke story that is generally and deservedly overlooked.