I took a quick look at the Compass Chemical Resistance Guide for Metals and Alloys and nickel plated ductile cast iron is "not recommended" for sea water or salt water at 70 degress F (not sure what the difference is between salt water and sea water!). Brass (unspecified) is rated borderline "not recommended" for sea water and salt water at 70 deg F. It seems neither is a particularly attractive choice. One tricky thing about corrosion by sea water is that it is often affected by velocity. Fatigue resistance and strength are important when selecting materials for high pressure pumps. You might want to look further and see if you can find a pump made of nickel-aluminum bronze or one of the duplex alloys both of which have been used successfully for this type of service.