How does the position of duck legs help the duck?

In certain duck species, the duck's legs are set far back on its body. This is especially prevalent in the merganser duck. These types of legs are not very advantageous on land, in fact, they make the duck "waddle" as it walks. But in the water, it helps them swim faster and with more control. This is the same with many waterbirds such as loons, grebes, and cormorants.