Why does George tell Lennie not to drink so much water in Of Mice and Men?

George tells Lennie not to drink too much water because it is scummy and he is afraid it will make him sick. The men have not yet reached the ranch, and it is standing water. You should not drink water unless it is flowing, as standing water is likely to make you ill. George is aware of this, but Lennie just sees water. He is thirsty, so he drinks. 

Lennie dipped his whole head under. . . "That's good," he said. "You drink some, George. You take a good big drink." He smiled happily.

George unslung his bindle and dropped it gently on the bank. "I ain't sure it's good water," he said. "Looks kinda scummy" (Chapter 1). 

The incident demonstrates Lennie’s lack of inhibitions and his childlike nature. It also shows how George looks out for and protects him, in ways big and small. If George wasn’t around to protect Lennie, who knows what would happen? George sometimes gets frustrated at having to look after Lennie so much, but he also enjoys his company.