Why do we have an ocean?

The question at first seems rather straightforward - we've got lots of water, and 97% is in the marine realm. Most oceanography texts tell us "the earth's surface is 71% water - we are "the blue planet." But little more attention is paid to why we have an ocean, as we know it. There is water ice on 19 moons of the giant planets, with a distinct possibility that the Jovian moon Europa may have a liquid ocean beneath an icy crust. Evidence from Mars suggests large volume of liquid water (lakes, small oceans) may have been present in the past, and large in channels and gullies on the Martian surface show evidence of liquid (water) erosion. Yet nothing in our solar system compares with earth's marine dominance. To make an ocean like Earth's several elements; are required: 1) A large volume of liquid 2) This requires elements forming the liquid be plentiful 3 The compound must have a prominent broad liquid phase 4) Gravity to retain the fluid 5) Depressions to contain the fluid if land and ocean are to be separate 6) An atmosphere 7) Luck - the proper narrow distance from a solar source. This paper will assess these requirements and demonstrate why the earth is unique in our solar system, and that life as we know it is dependant on these variables.