There is described a module for an underwater structure. The module comprises a plurality of walls defining a cavity configured such that at least two walls of said plurality of walls confront one another to provide respective stack support surfaces for supporting said module, said walls are substantially parallel to form complementary stack surfaces, such that one stack surface may rest on a surface and the other complementary stack surface provides a rest surface for another module. Two or more walls of said plurality of walls each comprise an aperture providing a conduit through each two or more walls to said cavity, respective apertures of said two or more walls and said cavity configured to provide a water flow path into, through and from said cavity. The water flow path may be considered unconstrained or at least substantially unconstrained in that water ma flow through the apertures and cavity without deviation or interruption other than by the module or cavity walls. In stacked formation an aperture may be occluded or partially occluded but still the module when on its own may provide water pathways which may be considered unconstrained or substantially unconstrained.
