A shear key unit for attachment rigidly to the base of a foundation raft for a maritime structure comprises an open ended thin walled cylinder, which when its open end is facing downwardly beneath the raft, acts as a fluid tight chamber of variable volume - the volume being diminished as the cylinder penetrates the seabed strata. The foundation raft can be lowered by controllably allowing sea water to escape from the chamber. A slidable closure may be provided at the distal end of the cylinder to isolate the interior of the cylinder from the seabed. It is a feature of the invention that the cylinder assembly can be installed through an opening extending downwardly through the foundation raft, and can thereafter be fixed in a position projecting beneath the base of the raft. A method of installing and attaching the shear key upon the slab is disclosed, and involves lowering the cylinder assembly through openings in the slab after the raft has been floated out, then fixing the cylinder assemblies in positions within the openings with the cylinders projecting beneath the slabs.