Being healthy is more about having the right ratio of cholesterol in your body than just having low amounts of the stuff in your blood. Specifically, it's beneficial to have higher levels of "good" cholesterol (HDL) and lower levels of LDL, or "bad" cholesterol.
Swimming can get these levels in the right balance thanks to its aerobic power, which has been proven to raise HDL levels. And for every 1 percent increase in HDL cholesterol, the risk of dying from heart disease drops by 3.5 percent [source: Bobalik].
What's more, studies have shown that aerobic exercises like swimming can also keep your endothelium in good shape. What's your endothelium, you might wonder? It's the thin layer of cells that lines your arteries, and it tends to lose flexibility as you age. In one study of people in their sixties who participated in aerobic exercise, however, endothelium function was found to be equal to those 30 to 40 years younger. The theory is that because aerobic exercise causes arteries to expand and contract, it keeps them fit [source: Bobalik].