Declaration of Water % use for material hydration

We are manufacturer of vegan products, water is used to hydrate the ingredients such as mushroom and textured soy protein to form the products. Following FDA requirements, do we need to include water as part of the ingredient list? If yes how do I calculate it, can I declare the water % based on the study data of the water absorption %?

Yes and yes

weight the ingredients you are rehydrating before and after rehydration to determine the water uptake. Once you know that, you'll be able to determine where in the ingredient listing you need to place the water

NOTE: if you're adding salt to the water at this stage, your calculation will need to take the % salt in to account as you may find that your product retains a higher amount of salted water