Ive not seem much difference between Asiatic islands (I'm originally from SE Asia) and the Carribean. So hopefully I can provide you some ideas. In coastal areas, I've seen lots of limestone types - but they do leach so it's something to think about when you need to plant around these things. We've also lots of granite types - usually more in the interiors. River rocks do work in an tropical garden, but usually we use it under or beside larger boulders.

Thank you, Ianna! How wonderful to have first hand knowledge of what I might use! If you are suggesting that the limestone leaches lime into the soil, well, that would probably be good because my soil here is quite acidic. I mean all the hydrangeas around here are true blue with no help to acidify the soil. Azaleas and blueberries grow with no amendments to the soil and seem to love it, so if I were to use limestone I could plant the neutral loving plants closer to the retaining wall and the acidic loving plants away from it. Am I understanding you correctly?

Thought I'd chime in. I live in South Florida, which is not totally tropical (we're subtropical) but we all tend to grow tropical plants (even pushing it to prove that we can grow those slightly temperature-sensitive plants that balk at our occasional 40 degree nights). I grew up in Brazil, have lived and worked in several South and Central American countries, as well as the Caribbean, and am real familiar with tropical stones because I'm in that business. Most areas there are alkaline, as you mentioned, or neutral. Here in South Florida we tend to use quite a bit of craggy coral rock, as developers blast it out every time a new housing development is built. And, more recently, you see more and more black or white Mexican pebbles or Indonesian jade pebbles, although not as retaining walls; usually as ground cover. In the Caribbean you see a lot of coral rock too. It is an integral part of our soil composition, and I expect there will be some very minor leaching over time into your garden, although not enough to changae your ph radically. FYI, in addition to retaining walls, tropical gardens often use stone as a groundcover or an accent as well. Besides the beauty of such tropical stones, they also serve the purpose of attracting and holding the heat from any daytime sun, and give it back off at night, sometimes saving a plant or two during an unexpected cold snap. Good luck to you; tropical gardens are such a joy!

carioca66 is right. I have visited three different regions of the Carribean and its all white coral rock. Neat looking rock. I wish it was available up our way .
Pet stores that deal in salt water fish sale this type of rock in crushed form. There are websites that teach you how to make coral rock from portland cement and crushed coral. You may want to consider this option.