The overall appearance suggests a copper or bronze alloy, so I agree that it is most likely a replica/copy, although whether it is recent or not is beyond me.
If you wish to make really certain, the coin needs to be properly conserved by a specialist in such things, perhaps a maritime museum or marine archaeologist would be able to advise you there; in the meantime if it was found in salt water keep it in salt water of approximately the same salinity, or, if you have to make it up from scratch, a ratio of approximately 6 parts salt for 200 parts distilled water will approximate the Caribbean in July. On the long-shot that it is not a fake, this will help protect the coin until it can be properly conserved or at least tested for its metal composition using XRF or a similar method.
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