Many were doing it to raise money for great causes, while others joined in just for the fun of it.
Mail photographer Frank Reid was in the thick of it to capture the occasion.
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1. Jolly good fun
Friends from Hartlepool and Stockton Health dressed as pirates and did the dip in aid of the Stroke Association and Harbour refuge. Picture by FRANK REID Photo: Frank Reid
2. Brass Monkeys
Just some of the 25 members of the Brass Monkey sea swimming group who joined in the Boxing Day tradition. Photo: Mark
3. Family tradition
Mum Liz Metcalf from Seaton Carew does the dip each year with her three-year-o,d daughter Harper and was 10-month-old Ferne's first. Photo: Mark Payne
4. Cancer Research colleagues
Members of the Cancer Research team who raise money at events across Teesside. They are on course to have raised around £250,000 this year. Photo: Mark