Why are the oceans blue?
The oceans are blue for much the same reason that the sky is blue-
the atoms that constitute the ocean absorb the other colors of light
more than they absorb blue.  Water molecules, which are made
from oxygen and hydrogen atoms, actually disperse a little more
blue than they disperse the other colors, they also absorb the other
colors a little more.  The effect is small and that is why small
amounts of water may not appear blue at all.  It takes a relatively
large amount of very clear water to obtain that slightly blue tinge.  

Often the ocean will seem like it is a different color because it
reflects the color of the sky.  This is why the ocean sometimes
seems red orange or yellow.  Also, high concentrations of plankton
can make the ocean appear green or cyan.  

Even if you can’t visually make the distinction, the water in a clear
glass, ice cube or fish bowl that appears perfectly transparent is in
fact a very subtle blue.