The whisker of a harbour seal is not round in cross-section. It is flattened and undulating along its length, a shape that breaks up the vortices the whisker would otherwise shed as it moves through water. Because the whisker is quiet in its own flow, it can register the far weaker disturbances made by something else.
In laboratory work, harbour seals wearing eye masks and headphones have tracked the wake left by a moving object well after the object itself had gone, following the path rather than simply swimming toward a sound. The whisker follicles are densely innervated and richly supplied with blood, which keeps them warm and sensitive in cold water.
What this does not settle is how much of a wild seal's hunting depends on it. Harbour seals also use vision in clear water and hearing over distance, and the relative weight of each sense in a real foraging dive is difficult to measure. The undulating whisker geometry is not unique to harbour seals either; it appears across several phocid seals.