You can make pumps with wood. Wood screw, wood block, wood pipe section. That's 3 from wagon.
Are there any good traps that use water?
Sure.
- Get invaders into 7/7 water. Anything needing to breathe dies. Usually done by luring them into a corridor, but there's no reason you can't drop non-fliers just straight into water with crossbow/ballista/minecart fire or lever pull. Or just dig out almost the entire map down to the aquifer, then cover it with bridges linked to lever (alternatively, with pumps make it bit higher and install drainage to recover gear).
Could make the room (temporarily) swimmer-safe with breathing room on level above, that can be removed by closing the bridge. Which conveniently also neutralizes anything not needing to breathe.
Note that massive digging efforts might need retire/unretire to work best.
- Powerless ammunition for watergun.
At high end, I think this is pretty much "anything dies" (expect perhaps a steel titan, vanilla, and I'm not sure there) though harder to setup and to cover entire map with. There currently isn't a public design that is absolutely perfect given the current understanding of minecarts, though there's several that can repel circus.
- Depending on map temperature, spontaneous ice tunnels/landmines.
Anything that isn't made of fire dies.
- Depending on map temperature, melting baths.
Anything that can bleed to death probably dies.
- Water pushing onto spikes/storage:
Not sure what the limit on this would be, but anything moved by water pressure should die.
It can also make neat things like trapped 7/7 water only shortcuts to get werebeasts (via a submerged minecart trap, for instance).
Probably lots other possibilities. Defensively, for instance:
- Moat: Rather eh, given the existence of drawbridges.
- Cleaning through: Useful when dealing with anything with syndromes:
- Water fighting grounds: Helps neutralize fire and syndromes (not sure about webs, wiki suggests they're destroyed by irrigation but water waves might be too slow).
PS: If you're thinking "wow water's awesome", then well. Main counterpoint against weaponizing flowing water is FPS. Why kill enemies with water if you can kill them without water?