This little incident is intended to show that George has to look after Lennie all the time as if Lennie were a small child. Warning Lennie not to drink too much water is obviously only one of the many things George has to tell him during the course of the day. George has been saddled with a responsibility that is wearing him down. He not only has to find work for himself, which is hard enough to do during those Depression days, but he has to find work for Lennie and then try to keep him from doing something that will get both of them fired. Lennie almost got them both lynched in Weed. It might be said that this little scene in which George warns Lennie about drinking too much of the stagnant water foreshadows the ending in which George finally kills his partner. It suggests that they have been together for a long time and that George must be getting stressed out from having to watch Lennie the way a father might have to get an eye on a six-year-old. Lennie never remembers anything and has to be told the same things over and over again. Lennie is compulsive, impetuous, unpredictable, and even potentially dangerous. George is bound to have outbursts of anger against him and then to regret them when he reminds himself that Lennie is mentally retarded. But Steinbeck was a realist and a naturalist. He didn't want to suggest that there was a bond of love between the two men. It was more like symbiosis. George is a little guy, and Lennie can give him physical protection in the tough world of unemployed, desperate men traveling around in boxcars seeking jobs or handouts. George, for his part, can give Lennie the direction the big man so badly needs. Without George, Lennie wouldn't be able to find a job regardless of how strong he was. Their relationship is not static. It has been evolving for both of them. Lennie is becoming more self-assertive, just like a growing child. George is becoming less tolerant, less patient, less understanding. He has a burden that none of us would want to bear, especially if we were having a hard time even supporting ourselves. They are not romantic characters. They are starting to dislike each other. That could be very bad for George if Lennie got to dislike him too much, because Lennie doesn't know his own strength.
An early clue to the relationship of the two men and to the character of Lennie Small is in their descriptions and mannerisms:
The first man was small and quick...with restless eye and sharp, strong features. Every part of him was defined....Behind him walked his opposite, a huge man, shapeless of face, with large, pale eyes, with wide, sloping shoulders; and he walked heavily, dragging his feet a little, the way a bear drags his paws. His arms did not swing at his sides, but hung loosely.
As they traverse the path, although he is the smaller man, George leads, indicating his dominate role. And, just as he stops, Lennie, who has already been described as bear-like, "flung himself down" and drank as an animal would, thinking only of his terrible thirst. George then warns him not to drink too much lest he get sick as he did the previous night. Obviously, the "green pool" has algae on it and the water may not be pure.