How many gallons of water are in the Mediterranean sea?

Answer 1

According to Wikipedia, surface area is; 2,500,000 km2 and average dept is 1500 m. (ie 1.5 km). Then the volume is 2,500,000 km2 X 1.5 km = 3,750,000 km3

Reference:

http://en.wikipedia.org/wiki/Mediterranean_

Answer 2

The approximate figures for the Mediterranean Sea are;

Volume; 3,550,000 cubic kilometres

There are 2.19969157 x 1011 Imperial gallons per cubic kilometre.

I leave the calculation as an exercise for the student.