Any matter have higher specific heat means it take much time to releases/absorbs heat into/from atmosphere.
water have very high specific heat once it get warm then it cools in long time.
in summer in the night water get cooled and in the day when temperature increases water absorbs heat and take much time to reach atmospheric temperature till the temperature of the atmosphere is decreases and the temperature around the water is almost constant.
in the winter in the day water attained the atmospheric temperature and in the night when temperature is fall the water temperature falls very slowly and the temperature around the water is almost constant.
that why weather seasons is almost constant near the sea.