1000 Years of Isolation.
About a thousand years ago (it’s a little fuzzy, give the historians a break it was a long time ago) something happened. What used to be a nice little port town, popular with tourists and a busy trading hub, became an isolated city. The sea turned to acid, boats and bridges touching open waters last for a few days before being consumed. Fresh water does still exist inland, in lakes, streams, rivers, and springs. However all water traffic from other lands has stopped completely.
All that is about to change, though.