Question: We know that the ocean depth is varies from ocean to ocean. Mariana trench is about 11 km deep. If we consider as a whole, what is the average depth of the ocean?

Answer: 
About four kilometers, see, for example:
<http://www.mbgnet.net/salt/oceans/data.htm> 
But bear in mind that much of the oceans, especially in the southern hemisphere, have scarcely been mapped at all, so it is currently impossible to give an accurate estimate.
From NOAA: (Official US Science) A recent estimate for the average depth of the ocean is 3,682 meters, or 12,080 feet.
This is an estimate from 2010. Older estimates have deeper average ocean.