It is shown how Truman has been made afraid to leave the island by having a fear of water created for him. Numerous other little subterfuges are revealed to explain why Truman has never left the island as well, from the ingenious to the hilarious. It's done perfectly. One, it is an important diatribe against our television mentality and, two, it is an ingenious recitation on the frailty of sanity. First, and foremost, is the theme of television.
American society is permeated by television.
All culture is now pop culture. The idea of a man trapped in a TV world not of his own creating doesn't even seem that insidious to us. In fact, it becomes a huge hit in the film and one wonders if anyone might raise any objection if the film's fictitious ruse were tried in reality. They don't understand that it is meant to shame them. Secondly is the precarious nature of sanity exposed here.