I am as thin as you, weighing 53 kilograms. I feel cold when I swim. But this requires an adaptation process. First of all, when swimming, you should do warm-up exercises to warm up your lower body. Don't be too violent Sweating is not good. When finished, sit on the shore, put your feet in the water and swing a few times, then reach into the water and pat your chest. That's it. When you are almost soaked, you can completely enter the water.

After that, you may feel cold in the water. At this time, you can try to swim several times as long as possible. As long as you move more in the water, the heat released by your body will be neutralized by cold water, so you won't feel cold. Therefore, don't just stay still in the water, swim more and hold your breath when diving.