Is there a white guy in the boat? If so, I'd be more afraid of that. My real answer is I'd scream some profanity and run away like I've been told I was the father of a kid I didn't want.
1st thought dinner.2nd thought new boots.
How to get my wife in the water before it swam away.
Your in a small boat, in the Amazon. There is no running away. Nor swimming...One could gather one's senses, and quietly paddle away. Therefore not causing attention to one's self.
I'd shout "The Hebrews were BLACK!" to that snake, scaring it , and then watching it slither away, continuing my journey.
It probably wouldn't do anything to you anyway. Snakes hunts by scent... he's looking for mammals and large rodents. Didn't you ever see those Anaconda hunts on Nat Geo? The snake does not attack you until you grab it.