Mark 16:18 "they will pick up snakes with their hands; and when they drink deadly poison, it will not hurt them at all; they will place their hands on sick people, and they will get well."
It's clear from the passage above that Mark is having Jesus speak of the physical harm that comes from being bitten by snakes, or contracting leprosy after touching the sick, not the "spiritual" harm that might come from someone who might "drink in" a false doctrine.
Thus, everybody would know that Mark is speaking of an actual, physical poison. God would have known that's what his readers would think, and that's why God didn't guide Mark in his writing of this passage.
Bang replies,
you imply that you don't even believe the passage….As I was asking, what did Jesus mean when He said, "..whoever drinks the water I give him”
Alward responds:
You’re right, Bang, I don’t believe this passage, because I don’t think a son of a God would claim that anyone who believed in this god could drink deadly poison and not be harmed. A real, all-knowing god would have guided his son to be more careful with his words, and not cause foolish but faithful readers to kill themselves by drinking poison.
Now, as for what Jesus meant when he said, “whoever drinks the water I give him will never thirst. Indeed, the water I give him will become in him a spring of water welling up to eternal life." (John 4:14): It’s obvious that the author means for the reader to believe that Jesus is speaking of the eternal salvation that comes to those who “drink” in the word of the Lord, i.e., accepts the teachings of Jesus. However, as I noted above, Mark has Jesus speaking of physical harm, not spiritual harm; thus, the “deadly poison” Mark mentions is actual, physical, poison, like the venom of the snake, or the virus of the leper, NOT the spiritual “poison” you have in mind.
Joseph F. Alward
"Skeptical Views of Christianity and the Bible"
http://members.aol.com/jalw/joseph_alward.html