You're right, ice is less dense than water and will float. The only thing I can think of is that before the ice is broken up, as it is melting, the top surface becomes wet making it appear as though the ice is below the surface of the water, hence people think it is sinking. In reality this water is just trapped above the ice until it starts to break up. If the ice did actually sink, you would never hear of docks being destroyed by wind driven ice flows. Come to think of it, if ice sank, we wouldn't be able to walk, drive, skate on it because it would be at the BOTTOM of the lake.
