I believe that Vietnamese people would be surprised if they knew Dutch people went swimming in the winter.
People in Vietnam 
prefer to stay home or at least inside when it is cold or it rains. What do Dutch people do in the winter? The weather is quite stable here: it rains heavily and a lot of wind blows most days of the week. Would you go for a walk? Do you want to go shopping or go out for a cup of coffee with your friends on these day? I would not join you.
Children will not like to go shopping with you either. I think. They will be not happy staying at home or inside throughout the winter. How can they burn their energy? Take them to the swimming pool!
You will meet your friends, your neighbors and maybe even someone who you like to disappear from the earth in the swimming pool.
Do not be surprised if you see many pregnant women or parents with their little children, not even toddlers, swim and play around you.
My first child went swimming when she was four months old. She liked it very much. The second child began went she was seven months old. Both of them like swimming.
Every Dutch child must have swimming diploma A. That is the
basic level. Many children have lessons until they have diploma C. My children are busy practicing.
I do not have any swimming diploma, luckily I still can swim. My family often goes swimming. Yup! Also in the winter.