Dear WW: I was a boy long enough ago that my parents could let me stay out until 3AM on moonlit nights during Christmas holidays. There was a twenty acre pond that had been created by damming a large brook to cut ice for sale long before refrigerators were invented. There was a large island on which we were allowed to build huge bonfires. I still remember the sounds the ice made, expanding until suddenly huge cracks would form to releave the stress. The specific gravity of ice would be depend on the temperature of both the ice and the water, and so would not be useful. Let it go that it is less than one.
Think of what the world would be like if ice sank. The oceans would be solid except for a film on the surface in summer. Life as we know it might be impossible.