In my last game I had a territory to the north and was looking for a location to place my harbor (should have grabbed a screen-shot but didn't).  While moving the cursor around I passed over a single tile of Sea Ice and the placement icon for the harbor stayed white instead of turning red.  I assume this is a bug, as you can't sail through Sea Ice so -- logically -- a harbor built on top of one would be permanently iced-in and useless, with now ships being able to enter/leave or units being able to embark/disembark.