Have you noticed those flat screen-like devices installed in the Olympic swimming pools? The screens are placed on the far-left end of the pool, facing the swimmers.
These screen actually are the Omega lap counters that appear to be underwater TV screens. These displays installed in the bottom of the pool, at the end opposite to the finish are used to show the lap counter to the swimmers.
These underwater lap counters were first used in 2015, at the world championships.
The square boxes on the finishing end wall are the touchpads used for automatic timing. The clock stops when a swimmer touches the touchpad at the end of the race.
You can see the touchpads in this video: