Did we ever record a reduction in sea level when the Panama canal was constructed?
It's important to note that major man-made navigational canals often are often routed so that only a smaller bit of new waterways is dug; they are typically made by connecting existing waterways with smaller cut\cuts across higher mountainous areas, plus shortcuts in any place where straightening the route saves significantly.
It seems hard to find detailed construction maps (unfortunate!)  But here is a map of the area before the canal was made from the USF historic maps archive:

Comparing that to the Wikipedia map of the Canal shows two main areas of new waterway creation

However, the largest creation on the map appears to be the expanse of Gatun Lake created by damming the river to create a wider channel (with other connected benefits like power and water storage).  Damming rivers is of course a widespread practice around the world for purposes including water regulation, power generation and agriculture.
Looking at rough estimates of water changes (hopefully with no errors!):
Note these use the US\UK definitions of billion as $10^9$ and trillion as $10^{12}$.  Replace billion with millard and trillion with billion in other regions.

The Culebra Cut &quot;has an average depth of about 43 feet (13 metres) and extends some 8 miles (13 km) to the Pedro Miguel Locks&quot; and &quot;throughout its length the canal has a minimum bottom width of 500 feet (150 metres)&quot; [Brittanica].
So that would be about 7 billion gallons (26 billion liters).
The Wikipedia articles lists Gatun Lake as 4.2 million acre*feet.
That's 1.4 trillion gallons (5.2 trillion liters).
Lake Alajuela is another 150 billion gallons (600 billion liters).

Given that the lakes are the significant amounts of water, 2 trillion gallons (8 trillion liters) would seem an extremely generous estimate for the amount of additional water impounded by the entire system (including widening the river and the water in the Gatun, Pedro Migual, and Miraflores Locks). If assumed all of that ends up being removed from the ocean, averaged over the 140 million square miles (362 million square km) of ocean surface area, that would be roughly 20 micrometers of depth (less than 1/1000th of an inch), comparable to the width of a human hair.  Again, remember that's a very generous estimate.
Given that Wikipedia lists 48 reservoirs around the world that each are more than twice the amount of the water estimated above, it would seem more significant to ask how much those remove in total from ocean.  Still, all those combined would seem only to be on the order of perhaps centimeters of depth lost in aggregate.