Is it cold to travel to Beihai, Guangxi in winter? Can you go to sea?

Winter is very long. The temperature in Beihai changes greatly in a winter, sometimes it is only seven or eight degrees, so it is cold to wear down jackets. You can't go out to sea in this weather. Sometimes it's over 20 degrees and you have to wear short sleeves. It is not cold at all, and you can swim in the sea. But generally speaking, we Beihai natives don't swim in the sea in winter, but dig snails and worms in the sea.