There are two reasons why George tells Lennie not to drink too much water.
First, George is like a father to Lennie. Lennie is mentally challenged. So, he is like a young boy, who does not know much about the world and the hazards of the world. So, when George tells Lennie not to drink too much water, he is looking out for him.
Second, there is a very practical reason why George scolds Lennie for drinking too much water. The water they come upon is not clean. George knows this, because the day before Lennie got sick from drinking too much of the water. Here is what the text says:
“Lennie!” he said sharply. “Lennie, for God’s sake don’t drink so much."
Lennie continued to snort into the pool. The small man leaned over and shook him by the shoulder. “Lennie. You gonna be sick like you was last night.”
From another perspective, George's scolding of Lennie is a beautiful thing, because it presupposes friendship.