We are constrained by the compositions of the early atmosphere and oceans that must have been such to allow it to evolve to its present condition, and they must have been able to sustain life for at least 4 billion of the 4.5 billion years of earth history. The earth formed from a cloud of dust that may have been unsorted or partially sorted. The outer part of this shell later remelted and separated into a heavy residue, the mantle, and a lighter primitive crust surrounded by an ocean and atmosphere. Carbonaceous chondrites contain more than enough water to create the oceans, indeed the problem is more likely to be where has all the water gone. There was considerably more volcanic activity on the early earth than there is today.
Volcanic activity is associated with release of volatiles, of which water is the major component.
The chemical composition of volcanic waters is similar to seawater so it has been suggested that the oceans were derived from pooling of volcanic waters. Filling of the oceans. The rate at which the early oceans formed is uncertain. Because volcanic activity continues today, it was first thought that the volumes of the oceans increased at a steady rate over geologic time. Long term sea level changes.