Every "science-type" website I've ever read with enough content eventaully brings up a page or two on water bottle rockets... of the compressed air variety.
Who's going to blame them? Mention a water bottle rocket, we instantly go back to fifth grade science projects (at least back when I was in the fifth grade, they're probably too mainline media now to do something so "dangerous"... but then again, we also built conventional powder-motor rockets in our tech lab when I was in junior high) and that one episode of MythBusters where they said "Don't try this at home" and built a metric fuck ton of them (including one made out of a 5 gallon water jug... those crazy TV personalities, right? ).
But if you followed the link above, you'll see that there is indeed another way to do this whole "water reaction mass" thing, which involves combustion by-products instead of compressed air.
In fact, one of the funnest little devices I've ever built was a simple 0.5 or 2L combustion powered water bottle rocket launcher.
I simply used a MAPP torch for fueling, a stick of 1/2" SCH-40 (which in case you don't know, fits perfectly onto the neck of a 0.5 or 2L plastic bottle (in the United States, at least), a schrader valve, and a long-nosed BBQ lighter to build it... it cost me literally nothing as I had the parts lying around already.
Here's the basic rundown on construction:
After this, firing is pretty simple. Fill a bottle anywhere from 1/4 to 1/2 with water, hose in propane or MAPP/MAP/Pro gas from a torch, cap it off or cover with your hand, and shake it up (to mix the gas with the air better). Then, simply stuff it onto the piece of pipe, turn it upright, and launch it.
These go a pretty respectable height for something built essentially from trash, and in fact have a lower peak pressure than the pressure most people fill their traditional compressed air powered rockets to. Take it for what it's worth.