He thought he could see concentric arcs outlining some of the farther peaks: pressure ridges where the ocean froze around the rock?

The oceans did not freeze.

If the temperature dropped below this value, the ocean would have subsequently frozen.

The ocean would have frozen after the moons escaped from the resonance.

More serious is that the oceans could perhaps freeze over during cold periods.

According to the Snowball Earth hypothesis, there may have been a number of periods when the Earth's oceans froze over completely.

It's another reason the ocean doesn't freeze solid in the winter; lowers the freezing point a few degrees.

Ponds, lakes and the oceans would freeze from the bottom up.

Phobos wanted to sell their know-how to the Martians, before their oceans froze.

Jake searched for an analogy and quickly found one--an ocean at twilight, storming, frozen into angular blocks.