When you move yourself through the water using your arms and legs, you swim. If you live near the ocean, it's probably a good idea to learn how to swim.
You might decide to swim across the English Channel, or, if you're a little less adventurous, just swim in your town's swimming pool. When things appear to be submerged as if underwater, they swim too: "I like it when the mashed potatoes swim in gravy." And when something appears fuzzy or whirling, it also seems to swim: "I was so tired the equations on the chalkboard began to swim." The root of swim means "to be in motion."