Some birds, such as ducks, have webbed feet to propel themselves through the water. Others, such as penguins, are highly adapted for swimming in the water and can use their wings as paddles (at the price that their wings are useless for flight).
Webbed feet improves their ability to swim. A duck has webbed feet so that it can push more water with its feet and therfore swim faster. Webbed feet help ducks move efficiently through water. Ducks have web feet to aid them in swimming along the surface of water.
Muscovy ducks can and will swim in a lake. If the Muscovy ducks are domesticated you should not allow them on the lake with other ducks because they can get sicknesses from wild ducks and it is illegal.
Certain kinds of water fowl can swim to the bottom of ponds and even some lakes depending on the depth. In the US there are 5 species of "diving ducks" the Ring-necked, Redhead, Greater and Lesser Scaups and Canvasback.