Because I don't trust you will ever actually run the numbers to back up your assertion, I've decided to give it a go, myself.
First off, we need to determine the volume of the atmosphere. The simplest way to do this is to determine the volume of the Earth and atmosphere together and then subtract the volume of the Earth.
The Volume of a Sphere is:
4 * Pi * (radius)^3
-------------------
3
We'll round Pi to 3.1416 for ease of calculation.
As mentioned earlier, the diameter of the Earth is 12,756 kilometers. Half of this is 6,378. To this we'll add the 150 kilometers of the atmosphere for 6528 kilometers. The volume of the who kit and kaboodle comes to 1,165,279,381,527 cubic kilometers (~1.17 trillion cubic kilometers).
Now, we'll work out the volume of the Earth, itself. Using a radius of 6,378 kilometers we come up with 1,086,783,833,910 cubic kilometers.
This means the atmosphere is 78,495,547,617 cubic kilometers.
Now, earlier we said one cubic meter of sea water supplies the oxygen for 3088.6 cubic meters of atmosphere. There are 1 million cubic meters in a cubic kilometer, so one cubic meter of sea water supplies the oxygen for 0.0030886 cubic kilometers of atmosphere. Therefore, it takes only 25,414,605 cubic kilometers of seawater to generate the atmosphere we have around Earth.
"Wait!" you say. "25 and a half million cubic kilometers of seawater is a lot of seawater." Let's see just how much that really is.
According to this Woods Hole site, the average depth of the ocean is 3.5 to 4 kilometers. We'll split the difference and say 3.75 kilometers. BTW, the above site is for grade schoolers, so it should be fairly simple to follow.
The surface area of a sphere is Surface Area of a Sphere = 4*Pi*r^2. Using this formula, we determine the surface area of the Earth to be 511,187,128 square kilometers. Water covers about 70 percent of this area, or 357,830,990 square kilometers. At an average depth of 3.75 kilometers, this gives us 1,341,866,211 cubic kilometers of water. It turns out you'd need to covert only about 2 percent of the Earth's water to hydrogen and oxygen to get the atmosphere we have. In 4.5 billion years, this has only lowered the oceans by 75 meters -- not a pittance, but certainly not anywhere near drying them up.
You know, these calculations can also be used to determine the amount of water required by Noah's flood:
"Fifteen cubits upward did the waters prevail; and the mountains were covered." Gen 7:20
A cubit was about 1½ feet, so the mountains were covered to a depth of 22.5 feet. The highest mountain in the world is Mt. Everest, of course, at a height of 29,001 feet. This means that the world had to be covered to a depth of 29,024 feet, or about 9 kilometers. We know the surface area of the Earth is 511,187,127, so we'd need an additional 4,600,684,143 cubic kilometers of seawater, or 3.5 times the total amount of water on Earth now.
See how absurd some claims are when one starts to run the numbers?
But a given size biomass is at an equilibrium for water release/consumption. (Burning carbos releases the water again.) You have to increase the biomass to increase the amount of water converted to carbos at a given time, and the loss is proportional to the physical volume of the biomass. (Much of which is in the ocean anyway and not hurting sea levels.)
I didn't check your numbers, but did you account for the fact that the oxygen in the atmosphere is very reactive and will also be removed? You know the "rusty" rocks that someone else mentioned.(not in those words)
Thanks for illuminating this question and going through so much trouble explaining it. However, I am afraid that the question has already become academic. Vade has been very helpful in debunking his own silly statement about plants creating the atmosphere on earth in post#673.