A modular dock unit and system for making large modular dock systems is disclosed. The dock unit includes a molded floatation shell, a molded decking piece and a molded dri-loc retainer ring. The floatation shell has column supports molded therein suitable for holding support members. In use, support member are placed in flanges molded into the support columns and the floatation shell rim. The dri lock retainer is placed on the side support members and the decking piece is opposed to the dri-loc retainer. Mounting aids such as tongue and groove members are molded in opposing pieces of floatation shell, retainer and deck and secured where necessary. The dock unit also includes a boat bumper system that mates with the dock units to surround the modular dock system. When assembled, the dock unit comprises a water-tight flotation cavity protecting the internal components and providing buoyancy to the dock unit.