Last February after a cool calm night, I took a morning walk to Totem Park and was interested to find that most of Crescent Bay was covered in ice. Salt water is supposed to freeze at 26F, and overnight lows probably got a little bit below that, but it was hardly frigid. Temperatures on other occasions have certainly been cooler without the extensive ice on the water that was present on this day.
When I arrived at the end of the park, I was able to see the ice extended all the way out to Eastern Channel, though I could not really tell how far out into the channel it went. On the other hand, I discovered that Indian River estuary had no ice in it at all. Given the fact that salt water requires colder temperatures to freeze, this seemed a little strange.
After some thought I decided the key to this mystery must be the calm clear night. I think the there was probably very little movement of surface water in the bay overnight. Clear skies allowed the surface to cool quickly, and since there was little or no mixing (perhaps partially facilitated by a freshwater lens on the surface?), the surface was able to freeze. In principle this would seem little different than making ice in the desert by digging a hole and placing an insulated pan of water in it that is exposed to the night sky. Presumably the reason the river/estuary did not freeze is the water was flowing and the resultant mixing prevented the surface from getting cold enough to form a layer of ice.