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Oh alright then as you asked so nicely

The job of the screwdown on the pushers is to prevent the operation of the pushers. This is done because the operation of the pushers while underwater compromises the water resistance of the watch.

In order to preseve the integrity of the water resistance rating it is only necessary to avoid operating the pushers underwater. It is best practice to screw down the pushers before going under water as that will prevent accidental operation of the pushers.

It makes no difference to the water integrity whether the chronograph is operational or not as long as the pushers were not depressed underwater.