Novel precast concrete boxes are disclosed which can be assembled together and/or with structural shapes disclosed in U.S. Pat. Nos. 5,697,736 and 6,491,473 to form waterfront structures such as seawalls, boathouses and the like. Novel methods of waterborne transport and installation of the boxes are disclosed, including modular vessels having bow and stern sections which can be connected directly together or mounted to a mid-section containing assemblies of concrete boxes, other structural elements or other vessels such as floating drydocks. Modular vessels can be assembled with any or all of the bow, midship and stern sections comprising honeycomb arrays of vertically-oriented hexagonal boxes.