A Portland-fly ash cement such as Blue Circle Sulfacrete is a good choice for concrete in these circumstances (look for the designation CEM II/B-V +SR) as it get progressively more impermeable with time and has a greater chemical resistance to the chlorides present in sea water. Sea water does not significantly attack good quality concrete, but it can cause rusting of any embedded metal which may eventually crack the concrete. Concrete exposed to sea water should always have a low water/cement ratio (below 0.55), high strength (C30 or above) and must be properly cured in order to develop its full resistance to seawater penetration. For reinforced structural concrete seek specialist advice.