water can be fairly useful. To drown, tire out, or beat someone, depending on the preference. Like filling the room with water, maybe with a current against them. Or blast it across the room to knock people down. Maybe both, fill the room and blast jets at them. Of course you could always put an octopus or jellyfish in it to make things interesting - though that sort of breaks your one monster or trap rule. An octopus could be trained to bring the unconscious victim to the master. Electric eel? Of course there's the classic shark, but that sort of bores me at this point. And again, don't know if this breaks your rule but chemically effecting the water in a slowly or rapidly filling room could be interesting. To poison or drug, immediate or later affects, dissolve clothes or equipment (dissolve metal? kill the swords and armor), whatever. Heh, sorry i ramble. i do that in brainstorming.
