Seawater density decreases as it freezes. Its density becomes less than the underlying liquid seawater and it floats at the ocean surface as a result. Seawater, in fact, undergoes both changes as it freezes. As water changes state from a liquid to a solid, its volume increases. This is one of the unusual properties of water.
Most substances are denser in their solid form.
Seawater also contains lots of dissolved and suspended materials that are mostly rejected from the solid form of water as it crystallizes. Most of the saltiness of seawater is omitted from the ice and is added to the underlying water. Not all of the dissolved substances are rejected, but enough is expelled to reduce the mass by a couple of percent. This decreases the mass of the seawater, and therefore also its density. Our solid seawater is less dense than the underlying liquid seawater and it therefore floats on top of the polar seas.