Quick sentences as such are great if already established the connection with the environment or the situation "John" has found himself in which would already be when just before this scene...
Darn-it!!! I just got more ideas in my head now!
I so want to use what I wrote and together with what you did just now... 
Even in light novels though there's more connection between words than short 5 word sentences, no matter the form a story takes every detail from future scenes are build in past chapters so if you couldn't establish that imagery in a readers thoughts before getting to these short sentences, it would just feel like "plotlines" or ideas rather than something your supposed to read in a novel.
So if to stick with what you did in short sentences it would look more like this if it was the start of the novel - before anything has been established yet..
The roar! The groans of the waterfall.
John grew worried, the speeding raft only fueled his growing panic!
He gripped tighter; he may well jump off!
Either way, the chances were just as optimistic...
But that's just prove more on the various styles of writing so really I'm not proving myself right nor you wrong only enforcing different means to the same end and that is story-building.