Why is it that water is cooler during night?

Because there is usually a lower temperature at night then in the day time.

Answer:

Water, during the day, picks up heat from the sun quicker thna it loses it to evaporation. Thus it stays warmer. At night the water tends to lose heat by evaporation as it ties to reach the wet bulb temperature of the air. This is usually cooler han the air temperature we record on a thermometer (dry bulb) so the water can get cooler than the air. The evaporation also cools the water (the hor molecules are the ones that evaporate).