I think it is a mixture of salt in the water and the movement. The north pole is actually a frozen sea. So it can freeze, but it is unlikely. When you add salt to water it changes the freezing point from the normal 0 degrees to around -10 degrees. So only if the temperature drops below -10 and the ocean is still, is it going to freeze.
Oceans do freeze but only when it’s very cold and then only the surface layers. We have sometimes had frozen sea near the shore in the UK, but the polar seas are frozen for much of the time.
If it wasn’t for the Gulf Stream (a warm water current which flows passed the UK) our oceans would be frozen more often in winter and the English Channel would be crossable by sledge or ice-breaking ship!
The salt is the main reason – it’s why we sprinkle salt on the roads and pavements in the winter. The freezing point of pure water is 0C, but the freezing point of salty water is lower, so unless it gets really cold (like this winter), the water won’t freeze.
The same thing happens when we make metal alloys – in many cases the melting temperature (same as the freezing temperature!) is lower for the alloy (mix) of two elements than it is for the pure elements. Solder is a good example – it has a very low melting point which is useful for making electrical circuits without damaging the delicate electronic components.
Salt does lower the freezing point of the water, but as they are so HUGE, it would take a lot to freeze the lot. You get surface ice in very cold areas, but to freeze everything would take something awful happening to the Earth, and we don’t want that!
As water gets bigger when frozen, I wonder where all the ice / water would go, if the oceans did freeze… I don’t think we’d be around to find out!