That's right. Water is most dense at about 4 degrees, and lower in sea water (around 1 C I believe). Below that, and water molecules start forming ephemeral cry

That's right. Water is most dense at about 4 degrees, and lower in sea water (around 1 C I believe). Below that, and water molecules start forming ephemeral crystal lattices. Since, weirdly, the hexagonal structure of ice lattices takes up more room than in liquid form, this makes the water less dense. Just to be clear, it isn't actually crystallizing at these temperatures, but due to fluctuations, crystal lattices can be briefly formed.