Liadan pure water kills rather quickly, a few glasses and you're practically a goner. Filtered water is similar but a bit slower and tap and mineral water don't really differ much methinks .. there you'll need a whole lot though. The mechanism behind water poisoning is that it can throw your internal ion balance, well .. off-balance.
Sangwa is also right.
"All substances are poisons: there is none which is not a poison. The right dose differentiates a poison and a remedy." - Paracelsus (1493-1541)
And I do understand why heroes don't use poison. It's because of the skill factor.
Take this exaggerated example:
Once upon a time there was a good guy. This good guy desperately wanted to become a warrior so he left his cosy farm home at the age of 14 and roamed the world in search of a teacher. A good couple years and teachers later he finally found a good one. The teacher he found was very hard on good guy and made him train day and night. Whether it rained, snowed or the sun scorched the earth, training went on.
Many years later good guy left his teacher and started roaming the lands again. He had a good few skirmishes, which he not only survived but also won. Quickly his name became more known and his fame gave him the ability to join any warlord or king around. Obviously good guy chose a honorable king to follow. In his king's name he quickly became a great hero, fighting for the people and all that. Some people were convinced good guy was invincible in battle and probably descended from the gods themselves.
Then one night bad guy, some lazy bum that wastes his life hanging around poor districts, getting women in his bed and being drunk whenever he has money. Now bad guy happens to stumble across good guy in a very drunk mood and he starts picking a fight. Good guy doesn't want to fight at first but obviously he can't let himself be beaten up completely so he defends himself.
Bad guy notices this, through a fog of alcohol, pulls a tube out, sticks a poisoned blowdart in it and blows it at good guy. Good guy gets hit in his arm and forcefully pushes bad guy aside, being fed up with this brawl. Then he continues down his path until a few days later he's found dead in bed.
The end.
Course don't look at me, I'm someone who doesn't like guns for the same reason.