[dredmorbius]

FINA competition pool standards have recirculation minimum standards, and pumps run continuously. Further, "water distribution has to be such that no appreciable current or turbulence is created. 'Appreciable current' is defined as water movement that can move a floating basketball (filled with 6 litres of water to obtain the right buoyancy) in one direction for more than 1,25m in 60 seconds."

<https://resources.fina.org/fina/document/2022/02/08/77c3058d...> (PDF)

Most competitive swimming pools have a large number of inlets with diffusers on them, laid out every 2m or so across the pool floor.

Those are circular disks about 10cm in diameter, looking vaguely like this:

  __________
  \________/
   |      |

    ^^^^^^

The carets indicate inlet water flow beneath the diffusers. The effect is that water entering the pool largely moves perpendicular to the pool floor, and slowly diffuses upwards. Water return is through the (large, wide, deep) gutters.

Because the gutters are continuously removing water from the pool, circulation needs to be on to maintain a consistent fill level.