Like animals, plants are aerobic organisms, and must also consume the sugar they make via aerobic respiration (C6H12O6 + 6O2 -> 6CO2 + 6H2O). So from just making and "burning" food, the mass of water is conserved by plants, less any sugar they store for later use.

(Backing up a bit, in photosynthesis, the oxygen atoms that go into the sugar come from the CO2, while the oxygen from the water (that makes 89% of H2O's mass) is released as oxygen into the surrounding air.)

On average the mass of water in and outside of a plant that isn't growing is in dynamic equilibrium, except for the changes in water temporarily stored in or moving through the plant (e.g., in sap). But for a plant that is growing, including just storing food, its overall mass increases. Most of this mass is carbon and oxygen from CO2, but also some is hydrogen from "destroyed"/"lost" water. (The rest of the water molecule is released as oxygen into the surrounding air.)

The bulk of a plant is composed of carbon, oxygen, and a bit of hydrogen. Some of this is sugar (food) that is temporarily stored for later use. Most of this is cellulose and hemicellulose, which are polymers (long, chemically bound chains) of sugar molecules, which comprise the structure of the plant. (Cellulose has the chemical formula (C6H5O10)n, where n is some big number of the C6H5O10 units. Note that C6H5O10 is a simple sugar, minus 2 H's and an O, or H2O. The combination of simple sugars to make cellulose actually releases water, so that somewhat reduces the net water consumed by a growing plant. But I digress.)

Switching gears entirely, there are many other non-biological factors that affect the amount of water on or above Earth's surface through goelogic time. It's not at all a trivial matter of whether the amount of water is increasing or decreasing through time, or at over a given time peirod. Volcanoes release water from the interior. Chemical weathering of rocks puts some of the water into the chemical structure of minerals. Subduction returns some of the water and "water"-containing minerals to the interior. Some water vapor is broken down into H and OH by ultraviolet sunlight, and some of those (especially the H) escape into space. (Comets and asteroids also deliver a bit of water and hydrated minerals, but beyond the very early Earth, this is negligible.)