The fishing trip represents a "normal" day for the men who are locked away from society in the mental institution. They went out, had some fun, caught fish and were spectacularly successful on their outing.
They gained self-respect and self-confidence by doing something that regular persons might do on a day off from work. They were forced by being together on a fishing trip to behave in a somewhat "normal" manner, so they left some of their "crazy-acting" behaviors behind. They were able to talk and be like regular people.
Upon their return, the doctor was able to successfully talk the boat owner and the police out of arresting them. Their success cost them as it only heightened the tension between McMurphy and Nurse Ratched. I sometimes think that this little bit of "normal" in the book shows up the administration in the mental institution as the "crazy ones".
In order to fully support these assertions, you will need to look up quotes in the text of the novel.