Marine concrete is an economical construction material for durable floating structures. However, marine concrete may experience severe damage during service in the ocean than inland environment. This paper assessed the durability of marine concrete exposed to actual ocean environment after 20 years of service. The chloride ions distribution and CO2 transport in concrete with varying grades were theoretically simulated using the past 22 years of weather records. The chloride penetration depth and concrete carbonation front were then analyzed and validated by field data. The results indicate that the chloride content and carbonation are highly linked to the microstructures of concrete and the weather conditions at service sites. As W/C ratios decrease, concrete usually becomes denser, and their pore structures are changed. Subsequently, moisture, CO2 and chlorides from ocean environment become more difficult to penetrate concrete than ever before. The results could be used to determine the critical values of harmful ions and the corrosion initiation time for steel-reinforced concrete structures in service under ocean environment.