This is because water is less dense the colder it gets (why ice floats) combined with colder weather cooling the top quicker etc.
Cold water is MORE dense than warmer water. I can see how one could conclude differently when ice enters the discussion, but cold water sinks.
Now, ice is a strange form of water. It is actually less dense than water, but not directly related to it's temperature, but more of the fact that it is frozen. Something to do with the hydrogen bonds pushing the molecules farther apart which makes it less dense than water.
I've exhausted my knowledge on the topic and should stop before I start talking out of my ears.
