Fish propel themselves by flexing their bodies. Flexing is active and is the source of the propulsion. There is some modification of the boundary layer by the compliant skin of the fish, the mechanism is not well understood.
Boats are propelled externally by a paddle. Here flexing of the hull is passive. It is a (small) energy sink - only important to people trying to go as fast as possible. Flex in the hull may be beneficial in multiple scenario, and Clyde appears to enjoy those benefits on the open ocean.
But a boat is not a fish.
Full Size Sail Rig
Touring Kayak Paddles
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