Comment by mojomark

I'm inclined to concur with onlypassingthrough. If the resulting wake is similar to fish locomotion (e.g. thunniform or similar) vortices will shed off in a Karmen Vortex Street that spreads laterally with distance behind the swimmer (potentially into other lanes, and propulsive efficiency of propulsors are generally less efficient in turbulant vice laminar open-water flow... but not always, it can depend on the 'structure' [how chaotic] the flow is).

The magnitude of the energy in that turbulent wake will depend on how efficiently the oscillating fin interacts with water over time to produce forward thrust. The cool thing about oscillating foils as opposed to rotating thrusters, is that when the fin 'swoops' once it creates Vortex 'A' spinning clockwise, and when it 'swoops' back the result would be a Vortex 'B' spinning counterclockwise, and the two vortices will partially cancel out. That cancellation serves to recover energy from Vortex 'A' and the energy is transferred back into forward thrust.

In other words, fish tails create trails of contrarotating vortices and continually push off of them. It's like walking up a springy staircase, where each step you make, a little energy is recovered to bounce you up to the next step.

In theory, if you had a swimmer in front of you, generating a Karmen Vortex Street and not effectively canceling out those vortices, but instead just shedding vortices, you can use the energy from the swimmer in front of you to 'spring' yourself forward - barely using any energy yourself. Those complex hyrdodynamic relationships could be why some swimmers/flyers tend to fly in specific formations with other animals in their school/flock.

Bottom line, I would bet that any residual vortices that spread into adjacent swimming lanes will tend to interact chaotically and result in unstructured turbulance, which should yield less optimal swimming conditions for swimmers in those lanes.