I think you're right about the rapids, and there are a number of other missteps - all minor in my view.
The rapids are just another element to make the story exciting - and a simple, if improbable, delivering the hero to his destination.
What *is* meticulously researched is the look and detail of the uniforms, the appearance of the trenches, the character of the Brits, the booby traps, the scorched-earth policy of the Germans (in Operation Alberich, on which the movie's set-up is loosely based, the German Army destroyed several towns and rounded up 125,000 French civilians to work for them behind their lines.
Mendes does a good job with language too. As a language researcher, I was pleased to note only one or two, momentary and unobtrusive, expressions that almost certainly were not in use in 1917.
There are a couple of other points one might whine about (and some have), but none seriously detract from Mendes's achievement (done with the help of his own army of actors and technicians).
Anyone who enjoyed 1917 will also like Saul Dibb's adaptation of Journey's End (2017) and, of course, Peter Jackson's documentary, They Shall Not Grow Old.