Nothing good ever happens when you get off the boat. Nothing. Zip. Nada. As Chef mumbles after his brief tiger encounter, "Never get outta boat."
If you're not attacked by tigers, you'll stumble into a massacre or get eaten up by the jungle like Kurtz. The boat is safety and "civilization." It keeps going upriver; it has the radio, their lifeline, that keeps the crew in touch with the rest of the army. Off the boat, there's only chaos.
Willard agrees with Chef:
WILLARD: Never get out of the boat. Absolutely goddamn right. Unless you were going all the way. Kurtz got off the boat. He split from the whole f***ing program.
Problem is, you can't keep the jungle away from the boat forever. Chief gets speared by tribesmen on the shore. Willard has to get off the boat to hunt down Kurtz and nearly gets himself killed. Chef ends up as one of the disembodied heads. When Willard finally gets back to the boat after killing Kurtz, it means he has rejected the savagery of the jungle and is returning to the order and the comfort of the boat, right?
Maybe not. When the command center voice crackles over the radio, he switches it off. Willard's learned that nowhere is safe, that the regular army's mission in Vietnam is an illusion.