Contradictory results have been published on the effect of sea sand and seawater in concrete as opposed to conventional river sand and freshwater. The majority of studies show equivalent, if not better, results when sea sand and seawater are used in concrete, although some studies concluded otherwise. This paper investigates the variations in the chemical and physical properties of sea sand. It has been found that each sea sand sample is unique, which might be the reason for the contradictory findings in the current literature. Chemical properties of sea sand were analyzed using a scanning electron microscope and the physical properties were observed using an optical microscope. The particle size distribution was measured. The short-term mechanical properties of sea sand–seawater concrete were compared with conventional concrete. The concrete samples were cured in natural marine conditions and atmospheric conditions and tested at 7 and 28 days. Sea sand–seawater concrete had a high early compressive strength at 7 days and remained slightly higher than conventional concrete at 28 days. The experimental procedure used all natural, unaltered, concrete constituents and curing conditions.