Clay boulders can be turned into "stone pots".
Don't they not hold liquid, though, unless fired? Meaning you couldn't brew drinks.
Anyway, my solution is pretty obnoxious and obscure, so I'll stop being an asshat and just say, hehe.
1) Embark on an embark location that has a cave tile, which leads down to the caverns without having to dig.
2) Build a carpenter's shop with wagon wood.
3) Build a training axe or two.
4) Go down to the aforementioned cavern, and harvest some blood thorns, which grow without cave water. As many as you'd like.
5) Gather surface plants (which are allowed, just a "no TREE biome") for food, farms, and distillable things.
6) make barrels and drinks and whatever else. You can also make walls and doors, etc. for defense out of wood. If clay boulders can indeed be used like stone, then you could even make mechanisms and bridges and whatever else? (Or can you only make pots out of them?) Either way, you can not only survive, but have a renewable wood resource you can make crafts out of and sell to build up exported wealth and fortress wealth.
Extraordinarily dull, but possible.
"Here is a scenario that *can* be done, and isn't impossible. Just almost so. Prove your cleverness by exploit and cheap trick to work within those rules to make an epic fortress."
Yeah like I said obnoxious. I wasn't really expecting anybody to take it very seriously. >.>