The main reason for oceans to be blue is the way water molecules absorb and scatter the colors of sunlight.

Sunlight has seven visible colors. They are red, orange, yellow, green, blue, indigo and violet. Now, when sunlight falls on the ocean, it passes deep into the water. Here, red, orange, yellow and green colors of sunlight are absorbed more strongly by water molecules. While, blue, indigo and violet colors are not absorbed much. Instead, they are scattered, making the ocean appear blue to us.

However, as the blue shades penetrate deeper into the ocean, eventually they also get absorbed. Hence, at greater depths the ocean is completely dark.