Totally get the emphasis on water—if you can't drink, nothing else matters much. But for me, I'd have to say my must-have is a solid fixed-blade knife. It's just so versatile: cutting cordage, prepping foraged food, whittling sticks for traps or even digging up roots in a pinch. Plus, you can use it to fashion other tools or start a fire if you've got the right kind of steel.
I guess I always look at how many different needs one item can fill, and a dependable knife has saved my skin more than once out foraging wild herbs or working in the garden. Don't get me wrong, I carry a little Sawyer filter too, but if I really had to improvise, I know a bit about boiling and primitive filtering.
Curious if anyone here has a certain knife they swear by for their kit? I've used a Mora for ages but been thinking of upgrading if there's something sturdier.