As the tidal current relaxes, the “U” begins to move in the other direction along the interface.
The “U”-shape disturbance radiates along the interface as an internal wave, accelerated by the incoming tide.
Anatomy of  an Internal Wave
Smooth Slick
Rough Waves
The internal waves’ upward and downward movements form corresponding alternating patches of rough waves and smooth slicks at the ocean surface.
Ebb Tide
Slack Tide
Internal waves in the ocean can form wherever a layer of less dense water sits atop a denser layer. Some “trigger” applies energy that lifts up or depresses the interface between the two layers, and that disturbance ripples as a wave along the interface.
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Flood Tide
Surface Effect
As the tide pushes both layers out, the lower denser layer dips downward when it encounters a dip in the seafloor, creatinga U-shaped depression along the interface.