The city is on the coast of a 14 tiles inland lake. All the tiles are coast tiles. The city have already build a workboat, lighthouse, harbor and the Colossus, but it can not build tiremes and galleys, while other cities next to the "real" ocean can.

Screenshots and savegame can be found in this thread (http://forums.civfanatics.com/showthread.php?t=252834).

I replied in that thread, but still:
the game requires a lake of at least 20 tiles for war ships and 10 tiles for lighthouse and harbor :)

Yep, just tested it myself and it's not a bug. Just a strange feature. :)