There are half a dozen turtle species that can be seen around the tropical Indo-Pacific coral reefs. While they look superficially similar, if you can get a photograph or a good look, you should be able to identify them.
The first step is to look at the lateral scutes (large scales in a line along either side) of the back of the carapace (shell). If the carapace has no scales, and the carapace is ridged, then that's easy - it is a Leatherback! (although these turtles are more commonly seen out to sea rather than around coral reefs).
If the there are four lateral scutes, then it is either a Green Turtle, Hawksbill or a Flatback Turtle. Also, the first lateral scute does not touch the nuchal (the small thin scute on the very front leading edge of the carapace, just behind the head). If the carapace is slightly upturned along the edge, then it is Flatback. The next step is to look at the head. The Green Turtle (and the Flatback) both have one large prefrontal scale along the front of the eye, while the Hawksbill clearly has two (as well as a more parrot-like beak).
If there are five or more scutes along the outer edge of the carapace, then it is either a Loggerhead or an Olive Ridley. Also, the first lateral scute does touch the nuchal. The Loggerhead has either five or six lateral scutes, the shell itself is elongated in shape, and the turtle has a large head. The Olive Ridley has a more rounded shell with six or more thin lateral scutes.
Chelonia mydas, 'Green Turtle'. The most widespread and commonly seen of the marine turtles.
Eretmochelys imbricata, 'Hawksbill Turtle'. Similar looking to Green Turtle, but with longer head and distinct 'parrot beak'. Note the two prefrontal scutes in front of the eye (split by a lighter edge) and the first lateral scute of the shell not touching the nuchal.
Esacus magnirostris, 'Beach Thick-knee/Stone-Curlew'.
Egretta sacra, 'Eastern Reef Egret' white morph.
Pandion cristatus, 'Eastern Osprey'.
Haliaeetus leucogaster, 'White-bellied Sea-Eagle'.