Seawater and unreinforced concrete
Concrete needs to be of a sufficient quality to resist seawater attack.
Information on the exposure condition for unreinforced (mass) concrete exposed to seawater is given in BS 8500-1: 2015 clause A.4.6 Table A.13.
Table A.13 gives the minimum quality of concrete with options for different cement types. For the majority of cement types (Table A.6 in BS 8500), the maximum w/c ratio is 0.55 and the minimum cement content 300kg/m3, assuming a 20mm maximum aggregate size.