You have been following the Life on Mercury thread? In there we have discussed the progressive loss of water from the planet Earth. Without meteors and soil erosion filling in the oceans the water level would be a kilometer lower than it is today! (a wild guess but it won't be that far out).
I suppose the reasoning is sound, although there are huge water reserves in the earth's mantle and crust in the form of chemical bonds in the minerals. As minerals break down, in some cases, through chemical erosion, additional water, oxygen, and hydrogen based compounds are released.
I'll see if I can find some specific minerals as references, but some crystals actually incorporate water molecules which bind the other elements and compounds.
Also, if you dissolve dolomite in an acid over a long time, you will create "new" water in small quantities.
Ok, Topaz is a good example:
Two moles of acid plus one mole of topaz should give two moles of water, eventually. Notice, I didn't say in human time scales, because in nature this could happen instantaneously, or it could require absurd time scales, depending on temperature, pressure, and saturation.
Gypsum is a perfect example:
Two moles of water per mole of crystal!
Under the right conditions in the presence of Hydrogen, Garnets could produce huge amounts of water by releasing Oxygen.
Although it's possible the reverse happened more often than not...
Perhaps water vapor in the upper atmosphere was destroyed by solar or cosmic radiation, with the hydrogen escaping, and then the free oxygen bonded to the rocks....
Anyway, water, or it's components, could be incorporated in rocks and minerals either as "inclusions" or "impurities" or as the basic building blocks of those minerals.
Both modern science and the Bible agree on at least one thing: The Earth was totally covered in water at least one time early in it's history, before continents existed.
The "excess" water must have gone somewhere, and that implies being included chemically in the rocks that make up the mantle and crust.