As far as I can tell FS is only ( and the only sea capable of it ) frozen on a weather score of 1 or 2,
"Lakes frozen, waterway, sea (if applicable) and rivers un-navigable due to floating ice in Cold if spring."
I've taken this to mean that only lakes actually freeze, while waterways, FS, and rivers are rendered impassable to both land and naval by unconsolidated ice. This in spring,
nota bene.
The terrain definitions say "Some parts of the Baltic and Black Seas are also frozen in winter (noted in army lists as FS)", which seems to be saying FS is frozen in winter no matter the weather score.
(Edit: Fixed formating error.)