normaly the reefs are few hundred meters from shore, at least in western coast, in eastern coast there is almost always too much wind and waves for kayak.
there can be several meters water in lagoons, but straight outside reefs there is very deep
but there is not reef all places and not much islands, from grand bay you can go by catamaran (1,5hours) to northern islands and there is nice places, but steams are strong, but coming to main island, or al least near of it.
i've been snorckling there and there is much different fishes in water deeper than 2 m and the water is clear, have some nice under water photos from there.
i've heard, that in south coast is someone , who is doing fly fishing there
and guiding customers too, but never been there my self, but next trip plan to go test it too.
if you are interested about big game fishing, send a private message to me, i can help you little