High specific heat means the substance takes a lot of heat to raise its temperature by 1⁰C. Also, water gives out a lot of heat when it cools by 1⁰C.
During winter water from oceans and rivers collects heat during the day and gives out during the night. The atmosphere gets warmed up in the night. It helps people from cold.
During summer because water takes a lot of time to heat up, water remains relatively cool compared to the atmosphere. That is why people swim a lot in pools and in rivers.
Water evaporates taking a lot of heat and goes up into clouds. That evaporation keeps the water on surface and atmosphere cool. Breezes from the rivers, oceans blow towards the land due to the pressure difference (created by hotter temperature of air & lighter density)