This may be a stupid question, so feel free to shoot it down.

Assuming all atoms have a magnetic moment, I would assume the water molecule too would have a resultant magnetic moment; ergo, it may be possible to magnetize a body of water. 

Can water be magnetized? 


Question: Is it a sin to do "works" for the sake of "rewards" in heaven?
Answer: No. It would not be a sin, necessarily, but simply an incorrect assumption. It may be a sin to teach this, however, because it is contrary to the teachings of Scripture, and you would be leading your brothers and sisters astray.

Consider what is said in Romans 3:27-28:


  27 Where, then, is boasting? It is excluded. Because of what law? The law that requires works? No, because of the law that requires faith. 28 For we maintain that a person is justified by faith apart from the works of the law.


If one has not faith, but only has works, it does not count for their salvation. That isn't to say that works are not important, as you can look at James 2:26:


  26 As the body without the spirit is dead, so faith without deeds is dead.


If one has faith, works will naturally follow. If one's faith does not produce works, then what is that faith? Our faith should be fruitful and produce works. This is actually analogous to the story of Jesus and the fig tree in Mark 11:12-14:


  12 The next day as they were leaving Bethany, Jesus was hungry. 13 Seeing in the distance a fig tree in leaf, he went to find out if it had any fruit. When he reached it, he found nothing but leaves, because it was not the season for figs. 14 Then he said to the tree, “May no one ever eat fruit from you again.” And his disciples heard him say it.


If we are the tree, and we have no faith, it will not produce fruit (works). If we are a tree producing fruit, however, God will bless us and we will feed many (figuratively or literally, depending on the work) with the fruit of our works.