High heat capacity of ocean water makes maritime climate less susceptible to temperature fluctuations between day-night / summer-winter.
Water has a very high heat capacity. This means it can take in/give out large quantities of thermal energy without an appreciable change in temperature. Thus water acts like a temperature stabiliser.
Oceans act like huge thermal mass. During the day time when the sun is up, it absorbs large quantities of solar thermal energy. After the the sunset when the land cools the ocean is still warm and this temperature difference leads to a flow of thermal energy from the ocean to the land. So the land near oceans do not cool much during the night time. So maritime climate is characterised by minimal variation in temperature between day and night / summer & winter. Continental climate has a large fluctuation in temperature between day and night / summer and winter.