Then you can at least dump it if your struggling with it. I used to have a 3 litter bottle wrapped in an old sleeping bag. Don't go for the breeze block option when you forget the water as I once did for a really hard session.
I am using water - but I didn't want to wimp out!
I wimp out every day of my life - it's ok!
PS. please discount me for Alps OMM - I'm clearly not likely to be fit