I see where you are coming from, and I understand your viewpoint.
Let me present to you my viewpoint.
From the beginning, There have been huge amounts of water, beneath the earth crust. this was not superheated water or anything. The earth crust, or large parts of it, were floating on top of this water so to say.
Whenever there is referred to the "great deep" these waters are meant.
Now, I ask you, to fill a bucket with water. take a lid, which fits tot he bottom of the bucket, and put it above the bucket. Now push with all your strength the lid down to the water.
See what happens? the water squirts up, without there being heat involved.
The sheer pressure of the earth on top of the water, was enough, to make the water explode to the only free direction there was, when a crack appeared between the great deep and the air above the earth crust. It exploded to the only direction available to relieve pressure and create an equilibrium in pressure.
Michael