I think that one element of significance regarding George warning Lennie not to drink the water is a practical element. George recognizes that if Lennie gets sick, they lack the resources and he lacks the capacity to provide care for Lennie. George alludes to this when he references how Lennie was "sick last night." George's role as caretaker is fairly clear in terms of how he is responsible for Lennie and in this capacity, he warns him not to drink the water. This helps to bring out another significant element in his warning. George is the "thinker" for both men. In this relationship, George is the one that has to do the reflection and calculation for both. Lennie is more of a child in this realm, and George is the parent. If Lennie does something reckless and careless, as a child would do, it is George's job and function to direct him properly. In the same light, Lennie's adherence to George's words are also evident as he shows a clear respect for George and what George says. This dynamic of their relationship is made clear from the opening moments the reader sees them together. This is seen in the first chapter in terms of the significance of George warning Lennie not to drink the water.
There is an allegorical level to this story that evades the reader if it is approached only on the realistic level. Because Steinbeck was a realist drawing photographic images of the itineret workers’ lives, it is easy to overlook the symbols: all our lives are a searching for a “paradise” of rabbits, abundant crops, “our own place,” freedom from economic slavery. We all encounter environments that are toxic to our health, whether foul water or work environments with a hidden infection. We all, like Curley’s wife, seeks companionship to escape our loveless lives, and that companionship is so often misinterpreted for desire or lust; we all have trouble balancing affectionate stroking with over-violent pressure, etc. And we all know the moral dilemma we may face at some time or another, the dilemma of seeing our loved ones suffer or compromising society’s rules to prevent it; sometimes, it is an old dog; sometimes it is a child we are rearing; sometimes it is an aging parent. If we are a collection of individuals, each working out our own problems, then the world is not unlike a bunkhouse.