How is water used during the light dependent reactions?

Water is split during the light reaction to form oxygen (${O}_{2}$) and Hydrogen ions (${H}^{+}$).

Explanation:

The light dependent reactions produce the oxygen that we breathe.

The hydrogen ions formed are super important for photosynthesis. They will be used to create ATP molecules. The ATP molecules will be used later on in the dark reaction (also called the Calvin cycle). The ATP molecules allow the plant cell to assemble carbon based molecules like glucose (${C}_{6} {H}_{12} {O}_{6}$).

Here is a video of a lab conducted to test how changing the $C {O}_{2}$ level available to a plant will change the rate of the photosynthesis. The lab focuses on the oxygen production during the light reaction.

This second video provides a more in depth discussion of photosynthesis including both the light and dark reactions.

Hope this helps!