Dangling like a giant maple leaf off the Corinthian Isthmus, just south of Athens, the 8,000-plus square mile Peloponnese might look like a behemoth, too big to tackle.

A nautical marvel, the four-mile waterway is located about 50 miles west of Athens and slices through the Isthmus of Corinth, a narrow landmass separating the Peloponnese peninsula from the Greek ...