Quick answer
The most powerful man in the country is not the last word. The verse says God can turn a king's decision the way a farmer turns water, and it says it about kings specifically.
Read both lines
The image is irrigation, which the footnote makes explicit: these are channels, cut ditches in a field, and the farmer opens one and closes another to send the water where the crop needs it. So the picture is not of water being created or forced but of water being directed, following the ground it is given. Applied to a king it says something enormous and something limited at once. Enormous, because the man nobody in the realm can appeal past is himself being routed. Limited, because the verse is about a king, in a book addressed to people who have no power over kings, and its comfort is aimed at exactly them.
What it does not mean
This is not a statement that every human decision is God's decision, and it does not make anyone a channel with no will of their own. The verse picks the hardest case on purpose, the one man whose choices look unappealable, and the whole point would collapse if it were merely saying that everybody is steered. Nor does it explain any particular ruler, past or present, as God's doing. A verse that comforts the powerless by saying the powerful are not final is being inverted the moment it is used to tell the powerless that whatever the powerful did was arranged for them. The book will say four verses on that a man's own way looks right in his own eyes, which is not the language of a script.
Related: Proverbs 16:9 · Proverbs 19:21 · Proverbs 20:24 · Proverbs 16:33
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.