To answer this question, we have to look at the difference between the temperature of water and temperature of the air both during the day and at night. When you go to the beach or the pool on a hot day, you'll notice that the water feels much cooler than the air. However, at night, as you've already noticed, the water feels warmer than the air. Why is the difference of temperature flipped? Well, both air and water receive heat from the sun during the day and cool down at night without the sun. However, air and water take in heat at different rates - air becomes warmer at a faster pace than water does. If we do a mental experiment in which we heat up the same volume of water and same volume of air, say if we were to heat up a 16oz bottle of water AND a 16oz bottle of air (Please DO NOT do this at home! The plastic will melt and destroy the stove), we will see that air only takes about one fraction of the time to heat to any certain temperature than water would!

If water takes four minutes to reach say 80 degrees, air would only take one or two. Because the water and air in the same place, say on the same beach, are exposed to sunlight for about the same lengths of time, water is actually colder than air during the day.

This same property of water - the fact that water takes longer to heat - also means that water takes longer to cool down, too. This property of water basically measures how fast heat can go in and out of water. Since heat goes into water slowly, heat also comes out of water slowly, and since heat goes into air faster, heat also comes out of air faster. After the sun sets, there is no longer a large source of heat for the water or the air. The air then loses heat very quickly while the water loses heat much more slowly, so we can feel that the water at night is warmer than the air at night. In more scientific terms, we say that water retains heat better than air does. Hope that answers your question!