coral for diorama.

just remember that coral reef arent one large rock, but the formation of the stony corals stacking on each other over hundreds of years. lots of holes, crags, crevices and odd shapes. you can't go wrong.

I would suggest google image search Tarawa and Reef to see what you can come up with for ideas. 

have 3 books so far plus looking at videos on youtube. i have ideas for four tarawa dios based on some photos and some descriptions. 

further research indicates the inner reef was not exposed. the water covering it was less than the 4 feet needed for the LCVPs and LCMs to get across. so, in my diorama (if i ever get to this one), most of the coral will be covered by the LCM ramp, whatever is driving off the ramp (sherman, stuart, or halftrack with 75mm gun), and some clear or murky water. so, given the small amount i will need,  i may be able to do this with some discount aquarium material.

I forgot to add that. Rarely will the coral be exposed becasue it cannot survive out of water, tides and waves keeps it within a certain distance from the surface of the water

you could show the coral by making the reef a lighter color than the surrounding area. Using google maps  and satelite mode you can see the effects on the color from reef