If ice is disturbed while growing, it forms irregularities. This means that it will not look as clear, because not all of the molecules were well-organized as they froze. There might be layers going through the ice that are not transparent, and the surface might be less smooth.
In the sea, there are almost always disturbances occurring during ice formation, because there are almost always waves. When ice forms in the sea, it therefore looks different from ice formed in your freezer, and sea ice forms through several different stages.
First, there is a stage when the water is very cold, but still liquid. Then, there is a zone where ice has started to form in tiny needle-shaped crystals that come together to form ice slush. The slush slightly calms the waves coming in from the open water, but there are still enough waves to prevent the needles from freezing together and forming a smooth ice surface.
Sometimes, on a calm day when there are no waves disturbing the process, needles can freeze together and a layer of ice forms.

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