Water has higher specific heat. A slight change in the temperature of water can release or absorb high amount of energy. When the surrounding air hot which is the case in summer, water can absorb heat from atmosphere and store it as internal energy by increasing its temperature and as a result, decreases the temperature of the surrounding. When the surrounding air is cooler as the case in winter, water can release the stored energy as heat and raise the surrounding atmospheric temperature. So due to its high specific heat energy, water can stabilize the atmospheric temperature.