Why are oceans blue?

Water acts like a filter to the colors of sunlight. The weakest photons (a packet of electromagnetic energy) are on the red end of the spectrum. In very shallow water, the weakest photons (red) drop out. Next, orange leaves, then yellow. At depths around 100 feet, the dominant colors that survive are blue, indigo* and violet.

Scuba divers have found they cannot appreciate the beautiful colors of fish and coral on deep dives unless they carry a light source because all but blue, indigo and violet have been filtered out. (Remember, color is caused by a reflection of light and without light, colors can't be seen!)

*Indigo blue is a variable color averaging a dark grayish blue that is redder and deeper than night blue.