You've asked if rhubarb and nettle leaves can be put in water and left to rot to be used as plant food.
The way to do it, Joey, is to find an old pillowcase and make a tea bag out of any leaves you have, rhubarb, nettles, dandelions, comfrey, anything really. Once the pillow is full, tie a knot in the end and use it like a huge tea bag - soaking it in a water butt. Please remember to keep a secure lid on the butt, and let it soak for a couple of weeks.
The resulting stew smells horrid, but dilute it 1 part to 3 parts water - it’s good stuff!
Remember to wear rubber gloves, just in case you are allergic to the contents.