Ducks don't have to mate in the water, but they are generally more successful mating in water. He is exhibiting mating behavior with the muscovy. At this point in the game, he would probably try to breed a water bucket if nothing else was available. You've already noticed how your hen's heads...
In retrospect, I realize that a disproportionate % of the offspring produced were drakes. I'm guessing that our fertility rate was around 80% with natural breeding. We kept five muscovy drakes with Approximately 20 pekin hens. The hens were selected from a flock known for natural...
The crosses will be sterile. They are also very good ducks for eating. Leaner than the mallard derived breeds and faster growing than their parents. As a kid we kept a flock of pekin ducks and muscovy drakes for the production of "mules" for the restaurant trade.