Why are oceans carbon sinks?
Check out the excellent Wikimedia picture of the carbon cycle:



All the numbers are in billions of tons of carbon: white = stored, yellow = natural flux, red = human contribution. Notice that the deep ocean stores much more accessible carbon than any other carbon cycle source, and is only surpassed by the lithosphere* in overall quantity of carbon stored on the planet. Some is stored by algae/sea plants, but the vast majority is stored as dissolved CO2.

*The lithosphere (crust and upper mantle) is not involved in the carbon cycle, and is therefore not shown in the above image.