[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.