If you want to lose weight by swimming, exercise is certainly good. If you are already thin, exercise will make you look healthier, not just so thin, but healthier. There are many factors that may be related to obesity. If you want to be thin, then you shouldn't eat less high-calorie things. Eat less sugar and fry less. You can lose weight. But if you are too thin to lose weight, then maybe your meat will become stronger, more fit, full of exercise and health. I think you can still swim. Strengthening exercise is a good hobby to pursue aesthetic feeling.