I beg your pardon? How much water has the Earth gained in the last 100,000,000 years?
Good question. It could be a thesis for a master degree geology student.
That means, it is a workable problem. We can model the annual budget of water on the earth and find it out. The earth has way passed its peak dewatering stage. But the new (cognate) water is still continuously coming up to the surface through volcanic activity.
The key problem is: we may not see all the water generated just by measuring how much water shows up. A lot of water are generated in the mantle but temporary stay and accumulated in there.