[Ibanez]

pawn Код:

new Float:x, Float:y, Float:z;
for(new i = 0; i < MAX_PLAYERS; i++)
{
  GetPlayerPos(i, x, y, z);
  if(z < {COORD}) // change '{COORD}' to your Z co-ord of when they are in the water or just above the water
  {
    SetPlayerHealth(i, 0.0);
  }
}

Could do something like that in a timer?