Because sea ice has air bubbles and saltwater pockets inside it, it is much harder to see through than non-salty, clear ice. This is because light can't go through it without hitting the bubbles and pockets. Most sea ice in winter is covered by a layer of snow, which you also can't see through. All this means that when the sun shines on sea ice, not much of its light gets through to the sea below the ice. This limits the amount of algae and tiny plants that can grow in the sea. This is important because when the ice melts, much more light gets through and the algae and tiny plants can then thrive. This effect also keeps the sea below cooler.