Cathbrighton, for the dehydration you'll need salts as well as water. I use Dioralyte usually, but in the past I've also made up my own with salt and sugar, following the online recipes
https://rehydrate.org/faq/how-to-prepare-ors.htmhttps://med.virginia.edu/ginutrition/wp-content/uploads/sites/199/2021/01/homemade-oral-rehydration-solutions-9-2018-1.pdfIf you're going to make your own do follow a recipe otherwise the concentration of salts won't be balanced.