I'm using a modlet that lets me craft water filters from the start without a schematic. This is the one I'm currently using: https://www.nexusmods.com/7daystodie/mods/3202?tab=description. I've modified the recipe to require 50 polymers, 3 metal pipes, 20 coal, 30 sand, and 3 glue. This change adds a bit of challenge and realism to the crafting process.
Please note that to be able to craft water filters from day one without needing a workbench or schematic, you need to remove the following parameters from the recipe.xml file: craft_area="workbench" tags="learnable,workbenchCrafting".
There is also the mod "Overrun Traders" that replaces traders with destroyed trading posts and adds water filters, solar cells to the crafting receipes: https://www.nexusmods.com/7daystodie/mods/4851?tab=description