Definitely more than a glass of water which was the original question anyway.

I'm not a scientist; but water is used in photosynthesis, which is probably the main chemical reaction it undergoes. [1] says that the C3 cycle consumes 120 GT of CO2 per year, that's 2.7267×10^15 mols of CO2. C3 carbon fixation consumes one mol of H2O per mol of CO2, so that's 2.7267×10^15 mols of H2O. That's 5 * 10^13 liters. The USGS says there's ~1.4 billion cubic kilometers of water on earth (who chose that unit?), compared to 50 cubic kilometers for the C3 fixation. That's a lot of glasses, but not many oceans. Perhaps a bigger factor might be from back and forth reactions in the oceans and atmosphere, but I'm not sure how to estimate those (e.g., water converting between different acids or getting hit by light in the atmosphere).

[1]: https://bio.libretexts.org/Bookshelves/Biochemistry/Fundamen...