This video of the frozen Baltic sea was shot around Rzucewo, Poland in January, 2010. I don't know to what depth it was frozen... just that it felt extremely solid. The ice was cracked and re-frozen in a number of places and there was a small layer of snow on top, 15 centimetres or less and there was evidence that someone had ridden at least one ATV (quad) vehicle on the ice in the last few weeks. The temperature in this video is around -15C and, with windchill, felt like about -22C. Because the Baltic's salinity levels are quite low it is easier for it to freeze than other seas or oceans but still takes colder and more sustained cold temperatures for it to freeze than bodies of fresh water.