The Miraflores Locks visitor centre is the best place to watch ships transiting the Panama Canal, and it is better than you expect. The viewing platform is close enough to the lock walls that you can look a container ship in its hull as it is lifted — or lowered — eighteen metres by the rising and falling of water in a concrete chamber that has been doing this job since 1914. The scale is not obvious from photographs: these are ships carrying thousands of containers, and they fit the locks with a few metres to spare on each side, guided by small electric locomotives called "mules" running on rails along the lock walls.
The visitor centre has a well-organised museum on four floors covering the French attempt to build the canal (1881–1889, which failed, partly due to yellow fever and landslides, partly due to financial scandal), the American construction under John Stevens and George Goethals (1904–1914), and the handover of the canal to Panama in 1999. Allow two to three hours including the museum.
The original locks — Miraflores and Gatún — handle ships up to the old "Panamax" dimensions. Since 2016 the new Cocolí Locks on the Pacific side and the corresponding Agua Clara Locks on the Atlantic side have handled much larger Neopanamax vessels. The Cocolí Locks are a short drive from Miraflores if you want to see the bigger ships; the viewing facilities are less polished but the ships are more impressive.
Miraflores is about 10km from central Panama City; taxi or Uber takes around twenty minutes outside rush hour.
The clearance between hull and lock wall is sometimes under a metre, which is why the pilots earn what they earn. Ships pass at all hours; the canal authority publishes a daily transit schedule, and timing your visit by it is the difference between watching three ships and watching none. Entry fee applies.