George and Lennie are the two main characters in Of Mice and Men by John Steinbeck, and even from the beginning we know that George has to take care of Lennie because he is unable to adequately take care of himself.
The two men have been traveling, and they stop near a deep pool near the river. George stops and wipes off his sweat, but Lennie
dropped his blankets and flung himself down and drank from the surface of the green pool; drank with long gulps, snorting into the water like a horse. The small man stepped nervously beside him. "Lennie!" he said sharply. "Lennie, for God' sakes 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. "
Obviously this is a common practice for Lennie, and George knows what happens when Lennie drinks too much water. It is a "green pool," so perhaps the water is not quite as fresh as it should be either, but it seems to be the amount of water Lennie is taking in that concerns George the most. It is a clear indication that Lennie needs George because he is unable to take care of himself.
I suggest that the water itself is of minimal importance to the story. What this dialogue suggests is that George has to watch Lennie like a parent, even though Lennie is a grown man who is actually much bigger and stronger than his friend. This is an easy way for Steinbeck to illustrate and dramatize the relationship between these two men. He doesn't have to explain anything to the reader. And it was important to Steinbeck not to have to explain facts in plain narration or exposition because he considered his story "a playable novel," i.e., a novel which could be immediately and easily transformed into the script for a stage play. Much of what would ordinarily be explained in straightforward prose in a conventional is explained though dialogue in Of Mice and Men. When George warns Lennie about drinking the water out of the river, the reader quickly understands that George is the authority figure who has tell Lennie everything, whereas Lennie is a childlike dependent who usually obeys orders. George has to watch his simple-minded friend closely, and it is dangerous to leave him alone, although George naturally wants to get some relief from the constant supervision, as he does when he goes into town with the boys and risks leaving Lennie on his own for a few hours back at the ranch. Lennie could not have killed Curley's wife if George hadn't left him by himself in the stable, thinking that Lennie was only out there playing with his new puppy. Lennie wouldn't have gotten into trouble with that girl in Weed if George hadn't left him alone for just a few minutes. So Lennie is a burden when Lennie is with him and almost as much of a burden when Lennie is not with him, because Lennie is likely to get in some kind of trouble if he isn't being closely watched. Lennie is likely to lie about what he has been doing while he was on his own. George is a substitute for his conscience, and he thinks it is all right to do "bad things" as long as George doesn't find out about them.
George warns Lennie not to drink too much water, because if he drinks too much, he may get sick, and will not be able to work at the ranch.
George is worried that if Lennie drinks too much water then he will get sick and not be able to work. in the beginning it also says that the water is not really clean water and it wasn't safe to drink.