You know that one of the ways to judge if a chisel is sharp is simply to look at its edge. If you can see the edge, the chisel's not sharp. Which means if you can't see it, it is sharp. Interesting, don't you think? The absence of evidence - you can't see it - provides the evidence - it is sharp.
Let's consider the circumstances:
Of all the ways to test for sharpness, why drop your sword in a brook? Which of them suggested this test? Why? Was it, possibly, the monk? Did he know what would happen?
So there they are, at the stream's edge, master and student. Was it a beautiful day? It should be. Let's let it be a glorious day, springtime, sun streaming through the trees, a serenity of bird songs and flower fragrances. The brook gently gurgles. Muramasa lowers his sword - I've heard it called "10,000 Winters", a bit chilling, you might say - into the waters. Leaves strike it and are severed. A gold fish (please, not a koi, please) errant on its way to supper, leans against the edge and is instantly decapitated. This is one sharp sword. A swallow skims the surface and... is split in two. A sword, for sure, for WAR. For victory!
Muramasa is elated. He spent hours, days, weeks, crouched over this sword, working it, perfecting its mettle, sweat-drenched and coal-fired, beating it, imbuing it with a warrior's heart - a sword should, must have such a heart - with one thought, to win, to prove its worthiness, to show the world, to show that old man...
Who, gently lowers his sword into the running waters.
A shy smile.
Muramasa waits. And nothing happens. Nothing.
Muramasa begins to giggle, a chortle, then an outright guffaw. He had nothing to worry about. The old man knows...nothing? Is that possible?
The fish swim by, uncaring, unknowing of their carefully tabulated significance. Leaves sail downstream, swirling into the future, unharmed. Birds peer down from a lazy breeze. Nothing happens. Nothing at all.
Minutes go by.
(You can hear the music, can't you? An angular banjo...)