Like the swimming pools he encounters Jim is slowly being drained. Young and naive, he has been filled with a sense of hope. However, having left the hospital, Jim is now alone and everywhere he turns doors are being slammed in his face. He has no family to rely on now and no one to take care of him. He is beginning to lose his innocence and, with it, his hope for the future. He will become so emotionally and physically drained that when Basie and Frank try to sell him to traders at the market that he does not even resist. He is being beaten down, literally and figuratively, by the war and—like the swimming pool—he is becoming an empty shell. He will not start to fill up again until he has some stability.
When Jim saw the drained swimming pool, he thought that maybe mass murder would be committedin the swimming pool. Apparently, the auhor wanted to show that the war had taken a toll on Jim. Looking at everyday things could also make his imagination run wild.
The swimming pool also served to remind that there was practically no one living in the house. Because, had there been someone living in that house, the swimming pool would have been filled with water. It also shows that the war had taken a toll of the residents in Shanghai. The residents near Jim house are all wealthy people. They had probably escaped to other countries.
In conclusion, the empty swimming pool just served to show that because of the war, many things do get affected and that people are affected too, emotionally and physically. It just showed how damaging wars could be.