OK, I will take it upon my humble self to explain this expression. I don't know why I have to do all the dirty work around here, but I do. Complaining? You think this is complaining? Wait until I tell you about all the fucked-up nodeshells I had to destroy today. You kids; I swear.
Now, try and picture death. What do you see? Oh, the skeleton with the black robe and the big scythe? Yeah, that's sorta what he looks like; but he's looking more like Calista Flockhart these days. Everything's getting feminized, isn’t it? Anyway, regardless of whether you see him as a bad man or a bad woman, the fact remains: He is dead.
Do you know what keeps you alive? It has a great deal to do with things that are moist. (And I'm not talking about how you got alive in the first place; shut up.) Your heart pumps blood, your glands salivate, you piss like a cow on a flat rock, etc. You're mostly made of water, they say. (It would be beer in my case, but that's pretty darn close, if you ask me.) So, when you are dead, there is no moisture. You dry up. Now, what would taste the worst if you were all dried up?
That's right! A cracker!
This, I fear, has been an exercise in explaining the obvious. All old sayings have a basis in truth. You need not look far to find it, usually.