"How could I possibly forget something that important that I've done a hundred times?"
Easily. That's why aircraft often have those big red "REMOVE BEFORE FLIGHT" ribbons hanging off all sorts of important things.
I'd also say to put the key on the raw water valve. Just buy a bright red neck loop for it, hang the loop next to or on the valve, can't miss it that way. Or...order a "REMOVE BEFORE FLIGHT" strip from your local pilot shop.