At the beginning of the story, something almost immediately noticeable is that the narrator, Joel, doesn’t state what exactly happened to his father to land him in the hospital. However, when the doctor says the phrase, ‘to keep his mind off his mind’ it suddenly becomes obvious that the father has some sort of mental illness. Furthermore, he refuses to believe that he has a mental illness, as he claims that if he takes his prescription medicine, ‘he’ll be too muddled to find the classroom.’ This seems to be a fairly common occurrence in older men with more prevalent mental issues, denying that they have any such affliction. In doing so, they surrender their family to watch as their behavior becomes more and more abnormal (and a lot of times, self-destructive), all the while denying that there is anything wrong. Even when they are trained in a science that advocates taking medical supplements when finding oneself with a deficiency.
Over the course of the story, it becomes clear that there is a change overtaking the father, for example changing church affiliation and wearing scuba gear around the house. At one point, Joel follows his father and watches what his ‘church’ does. They engage in a strange ritual, that includes the pastor holding a disgruntled rattlesnake and Joel’s father speaking in a strange tongue. Joel’s father meets with him afterwards and tries to convey why he chooses to go to church instead of following the medical treatments recommended to him. Joel remains unable to understand, and his father tells him that he hopes Joel never does. However, he eerily mentions that the doctor at the hospital had hinted that Joel may be subject to the same type of depression as him some time in the future. For some reason, Joel never tells his mother about his venture. Perhaps out of respect for his father’s privacy. Or perhaps because Joel doesn’t want his mother to start worrying about him as well.
Towards the end of the story, after Joel’s father passes away, Joel continues to venture down to the lake house where the death occurred. He and his mother were told that because of nitrogen narcosis, his father removed his mask while deep underwater. Joel contemplating whether or not this was actually the case–whether or not it was actually planned on his father’s part–may indicate that he is beginning to understand his father’s thought process.
I think the complex relationship between father and son is what makes this story so alluring. Joel’s constant strive to better understand his father only to find the remains of a man he can’t seem to understand is so integral to the characters growth as a person. I believe the ending, as you said, is Joel finally coming to terms with his father illness and the fact he may also share the same problems. He is coming to see why his father acted the way he did. Whether this is due to Joel developing the same illness or just the ability to look at situations from an outside perspective that comes with the age and maturity is unstated. I believe this shows a beautiful understanding of mental illness and not only its solitary effects on an individual but also the people around them.