A short distance into the tunnel, they'll be able to see something inscribed on the left-side wall. A closer inspection will reveal small movable panels with patterns on them. It appears to be a picture-based puzzle that requires sliding the panels around to form matching patterns.

It's a little complicated, but anyone with a good head on their shoulders should be able to solve it within a few minutes. Though, that means several minutes of standing still in the cold water, painfully aware of the dark abyss looming up ahead...