Compared to rocks, ice is not that dense. You can try yourself: it's easy to pickup a big chunk of ice, even with a shovel, while a similarly sized rock is impossible to lift.
"Other substances that expand on freezing are acetic acid, silicon, gallium,[35] germanium, bismuth, plutonium and also chemical compounds that form spacious crystal lattices with tetrahedral coordination."[0]
There's more than water. A lot of things that freeze into a crystal lattice can be less dense than their liquid states. Silicon dioxide is another IIRC.
