A person cannot drink salt water and survive for 16 months at sea...so unless he could catch enough rainwater to drink over that period of time or store some between rainfalls, plus catch a lot of fish needed to keep him alive, it’s not possible.
Water is staff of life and one must have some every 3-4 days. If he had the storage for water, and fishing gear it might be possible??....Some skepticism here on the length of his lost at sea experience.
I’m not buying his story, either.