Swimming has two parts–one is to keep up in the water, and the other is to move along in it. The question really is: Why is it really easier to keep up, or float, in saltwater? The answer depends wholly on the heaviness of our body as compared with the heaviness of the water. Our body is more than three-fourths water. The fat of our body is lighter than water, and so helps us to float.
Freshwater is less heavy than saltwater, and so our body, though only a little heavier than it tends to sink in it. Ordinary sea water is heavier than fresh water, because it contains a lot of salts dissolved in it, just as the water of our own body does. So we find it easier to float and swim in sea water. But in some part of the world there is water that is much saltier than even sea water. This is the case, for instance, in the Dead Sea, and the Great Salt Lake in America. There is so much salt in the water of the Dead Sea that it is actually much heavier, on the whole, than our body is, so you can not sink in the Dead Sea! (See photo above.)
On the other hand, there are some liquids much lighter than water, and if a man were to fall into a lake of one of them he could not swim at all, however good a swimmer he was, his body would sink like a stone in such a light liquid.