Two reasons:
1 - Convection (the warmer, lower density water stays on top)
2 - The sun - solar radiation heats the surface of the water and is absorbed, so it cannot penetrate deeper
they want to
This happens because your body temperature gets used to the cold water, at the same time the temperature from the body makes the water a little warmer also.
I have found one in my outside fish pond, and he seems to be doing fine, crawling around the bottom. Doesn't seem like it wants out of there.
Water retains heat longer than the air does. As a result, at night when the temperature drops the pool is fairly close to the temperature it was during the day while the air around it can become significantly colder. The same effect occurs with the ocean where the warmest ocean temperature on the eastern US are in September when the relative air around it is much colder. Pools seem warmer at night because the air outside (ambient air) is normally cooler. The temperature of the pool at the end of the day is when its the warmest and it cools down overnight. So ususually by morning the water is not as warm as it was when you get in earlier during the night.
There seem to be several:νύχτα: night, night timeβράδυ: nightνύκτα: nightνυξ: nightβράδιά: night
because it is the end of the day so many people return from work to enjoy there evening with there partner so it's getting heated making it warmer.
Phases
Things may seem louder at night because there is less ambient noise to mask the sounds, making them more noticeable and prominent.
part of it is perception and part seasonal. As the temperature warms snow at the higher altitude begins to melt and flow into rivers and causes the rivers to be colder. When fall and winter come about the snow has all melted and the river has been heated by the sun making it warmer. Now for the perception part. As you are standing on the bank in the heat of summer the water feels cool and refreshing to your heated skin. In the winter your body feels colder so the temperature difference doesn't feel as extreme to your sense of touch making the water seem warmer than you remembered it being in the summer.
It seemed you were busy last night.
Predation pressure is lessened at night, but they seem to feed at night when molting, as well.
When you look at the stars at night, you can't really tell how distant they are. They can easily seem to be closer than they really are.