An alternative, based on my findings and suggestions in the discussion. Personally I think what matters here is the shape rather than what it's made of, and I would be happier including a nod to the actual (apparent) meaning of "ragues". Actually artificial reefs tend to have holes rather than crevices, so I don't think it's a tautology.

"Layer" and "tier" would do equally well.

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Note added at 20 hrs (2018-05-18 09:06:32 GMT)
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It is not a common word, clearly, and I think it's almost certainly regional (south-west); Provençal origin would fit that. But it certainly seems to be standard in diving circles. If you look for "ragues" + "sous-marin" you find quite a lot of examples. And I think the meaning is definitely crevices, specifically horizontal ones: deep, narrow cavities where fish and crustaceans lurk.

I still think that because the object of this particular artificial reef is to create a habitat with crevices, the emphasis should be on the "ragues": the gaps between the slabs rather than the slabs themselves.

Here's the French version of a document on the best Spanish diving sites. It uses the word "ragues" three times, referring to the Catalan coastline, and each time it's translating the Spanish word "grietas", which means cracks or crevices. For example:

"Les parois, tapissées de belles gorgones, possèdent de nombreuses ragues qui abritent congres, murènes et langoustes"
https://issuu.com/estacionesnauticas/docs/les_meilleures_des... (p. 2)

Spanish version:
"La pared, tapizada de bellas gorgonias, muestra numerosas grietas con congrios, morenas y langostas"
https://issuu.com/estacionesnauticasdemenorca/docs/30mejores...

This is about a wreck colonised by fish:

"Au printemps, des bancs de sars et de saupes [...] dans les interstices des carcasses, formant alors d'immenses ragues"
https://books.google.es/books?id=hbsTCwAAQBAJ&pg=PT246&lpg=P...

Another indication that it's specifically a horizontal crevice:

"Suivant le type de roche ou sa configuration, on peut parier du type de poisson que l'on peut trouver. Ainsi, ragues horizontales et autres failles verticales ou obliques sont des endroits privilégiés pour le sar."
http://magieweb.pagesperso-orange.fr/plonge2.html