can i knockback saomeone in the water to kill it immediatly?

No.

(extra text so the brain dead forum software will allow this post).

No you can not.

Knockback: After inflicting at least 1 damage (before applying the effects of armor) to a figure with a Knockback attack, the attacker may immediately move each affected target figure up to three spaces away from its current location. The figures must be moved to spaces that do not contain other figures or obstacles that block movement. The figuredoes not actually move through the first two spaces - it is knocked completely over them. As such, this “knockback movement” is not blocked by any intervening figures or obstacles (though a figure cannot be moved through a closed door or wall).

Water blocks movement.

Even if water didn't block movement (or Knockback didn't have that restriction), there is no rule saying that entering a water space results in instant death. This question keeps coming up, but even if people don't read Knockback carefully, I don't understand why people keep suggesting that water = instant death.

Its because of that joking reference that was once made about the reason you can't jump over water spaces was because of bloodsquids lay in wait in the FAQ

Q: Why can’t I jump over water like I can pits?
A: As every hero knows, the dank water found in subterranean caverns is usually infested with deadly bloodsquids, ready to pull an unwary hero to his doom at a moment’s notice. As a result, only the most acrobatic heroes dare to leap across even the most innocent-seeming pools of water.

Its a Descent urban legend if you will.