To my knowledge, they didn't. The Adriatic sea is a relatively warm sea and I believe that the salinity, which is slightly higher than in the rest of the Mediterranean, further prevents freezing. Also, the air temperatures rarely fall under the 0C. There is, however, a strong northern wind called Bora or Bura, depending on who you ask which can make you feel as if it were below zero, but to my knowledge it has never contributed to anything save a very bumpy boat ride.