Water has higher specific heat. And for that reason, it can act as effective heat bank for our atmosphere. A small change in the temperature of water is accompanied with release or absorption of high amount of energy. When the surrounding air hot(in summer), water can absorb heat from atmosphere and either become vapour or just store it by increasing its temperature and as a result, cools the surrounding. When the surrounding air is cooler(in winter). water can release the stored energy as heat and raise the surrounding atmospheric temperature. So water helps in maintaining a balanced temperature of the atmosphere and it can do so due to its high specific heat energy.