Bean says : always thought it meant a major, but slow, change. (This probably has something to do with my work in physical oceanography...)

Got a surprise with this one! I've heard and used the phrase since I was a lil' one ... used around the New England seacoast meaning the weather has changed and a cool wind is coming off the sea ... specifically it's a Summertime word used when the weather has been hazy, hot, humid (3H weather) then there is a sea change and the wind comes across the cold ocean waters to cool us all off. A sea change is a welcome relief ... could use one today!