Ice weighs less, explained well by @ragingloli.
When I freeze liquid in a bottle, I always make sure there is some space for the liquid to expand into.
Side note: As far as floating. When I was a kid it was so much harder to float, especially my lower legs would sink. Now that I’m fatter I float with no problems. Lol. I remember learning how to tread water for safety in the water, and now I’m not even sure I can sink if I just lie on my back.