Sea water is alkaline mainly due to the presence of appreciable quantities of carbonate, bicarbonate and borate ions. These react with the H+ from the ionization of water leaving an excess of OH' present….
West, Lloyd E., and Rex J. Robinson. 1941. "Potentiometric analysis of sea water II. Determination of titration alkalinity." Journal of Marine Research 4, (1). https://elischolar.library.yale.edu/journal_of_marine_research/581