[defrost]

If it's too shallow the swimmers arms hit the bottom ...

Slightly deeper and there's drag from the floor as their arms barely miss it. That effect persists until it doesn't .. now it's deep enough.

It needs to be deep enough that vortex's created by swimmers have disapated by the time they reach bottom and reflect back to the surface so as to not interfere with following swimmers or swimmers returning.

[sshine]

> If it's too shallow the swimmers arms hit the bottom

Is it’s deep enough, the gravitational mass of the water will form a black hole and squash the swimmer to death.

[defrost]

    Big whorls have little whorls
    Which feed on their velocity,
    And little whorls have lesser whorls
    And so on to viscosity.

~ Lewis Fry Richardson

    and then to an event horizon.

~ sshine

[expression1sh]

Well before that the hydrogen atoms would fuse into helium and release vast amounts of energy, killing all the swimmers, spectators, and many people in Paris.