This is a scene from the novel that is difficult to visualize, even today: patients from a mental ward getting on a charter fishing boat and spending the day out on the water. This is part of McMurphey's struggle, almost from the moment he enters the institution, to convince the other inmates that they are still human, still men, still deserving of some of life's simple pleasures, and deserving of being out of Nurse Ratched's iron fisted control.
This act, however, is more than simple rebellion on McMurphey's part. It is perhaps the most normal event in the entire story, and provides a stark contrast to the daily life of the inmates. It is also a humanizing event in the eyes of the reader, where we are allowed to forget about both the mental illness and the abuse suffered by the characters in the story.
To chime in with brettd, I think this is a normal scene, but it's also vey steeped in Biblical allusions. Before they leave, one of the characters in the hospital pulls his hands off the wall and tells Mac to go and be a fisher of men. It's also important that it's not just the patients that are there, there is also the male Dr. Like the men on the ward, he has been emasculated by Ratched and the domineering women in the novel. This is his opportunity to become a "real man" as well. There is also the image of the men fishing for the leviathan of the deep. They are battling with this great evil, you could consider Nurse Ratched, or maybe even Moby Dick here, and the men are able to land this huge fish that impresses everyone. They have gone out and conquered the archetypal female image (the sea) and come back stronger for it.