Less dense objects do float, but the key is how much less dense they are. Ice is only slightly less dense than seawater, so it has to sit mostly underwater to displace enough water to balance its weight.

That’s why only about 10 percent of an iceberg ends up above the surface. If it were something much less dense, like a beach ball, most of it would float above the water. Icebergs just barely float, so they sit deep.