Swimming can be considered a universal sport. Seniors, overweight people, pregnant women, people suffering from chronic back pain ... everyone, or almost everyone can swim. Buoyed by water, the body is lighter, thus facilitating the movement and relieving the joints. At the same time, resistance to water makes the muscles work harder.
Generally the fitter a person the quicker they can reach the last gear, also a fit person can exercise at a slightly higher level of effort and still burn mostly fat. An experienced swimmer has had many years to develop fitness. It means that the energy system can easily cruise at a level where muscle cells can take full advantage of fat stores for fuel. If all people had equal abilities then swimming would be ideal to lose weight, the trouble is we all have different levels of fitness.