A precast, modular marine structure and method of constructing the same for offshore use, including but not limited to drilling, oil and gas production, and oil storage in a variety of water depths. The marine structure includes an equalized pressure system and concrete modular components cast with at least one cell and a central longitudinal passageway. The equalized pressure system fluidly connects the cell(s) to the adjacent body of water by at least one substantially vertical segmented water column to equalize the hydrostatic pressure differential experienced at a wall of the marine structure. A truss section may be attached to the concrete portion of the marine structure to form a truss spar. A mooring and tether system may be included to maintain the marine structure's station and attitude. Construction of a marine structure includes assembly line techniques to form and cast individual modular components (such as a segment or module) in a position which encourages the pouring and curing of a concrete slurry; slipping the modular component from its form; translating the modular component into a position for mating with other modular components; and mating and connecting modular components with tendons to achieve a unitary marine structure.