[bug] The trouble is connected to specific beaches.  I managed to get a unit into a ship (though I was unable to do so at a city with a harbor), but then I could not get that same unit out to explore islands.

So: beaches where I have built a harbor are generally (though not always) undockable, whether or not I try to dock the ship directly into the city or to empty sections of beach.

[Suggestion] I would like to have larger sections of beach.  While I'm able to build a fishery on one area, I'm not able to later build a harbor. That can be a nice balance, but since the fishery is required for  harbors, I would have to demolish it to move onto the next stage.