If you build machines like on the screenshots, they will move forward (for the first one you should disable gravity to fly). This means impulse of water is much bigger then return of cannon. It can't be true in real physics world.
I can't decide if this is a bug or it was originally intended.
Normally the kickback of the water gun's propulsion of water would cancel out the push made by the collision of the water and the wood blocks.
C'mon this is just rocket science people XD