• The Culebra Cut "has an average depth of about 43 feet (13 metres) and extends some 8 miles (13 km) to the Pedro Miguel Locks" and "throughout its length the canal has a minimum bottom width of 500 feet (150 metres)" [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.