New Sea Ice forming
When sea-ice starts to form it can do so over 10's, or even 100's of square miles of open sea at a time. The secret is not only cold conditions but also a calm sea. Any amount of movement over the sea sweeps the forming ice to one side and prevents it from solidifying, conversely a calm sea can lead to what was all blue and bouncy the night before being white and spookily flat the next day.
Photo; © Mike Usher - Pictures from an icebreaker cruise to Eastern Antarctica.