Edited by MikeyLikesIt - 12/27/10 at 9:38am
This was my first year to have ducks and I had the same questions you have. Muscovies are the only breed I have. I bought them for meat but they have become my wife's pets (she thought I was nuts to want them, but now, if we had to choose between all our animals, the muscovies would be the ones she would keep). And the ducks love to 'work' the garden with my wife.
We have two ponds, both smaller than yours, that our ducks stay on. We have no shelter or pen and have lost 6 to predators. We started with 12, however, two of the 6 we lost had contracted Avian Botulism, or "limber neck", which made them easy prey (I had them quarentined in a cage but a bobcat tore through the chicken wire...hard lessons learned) When we first got our ducks, they were young and still flightless. They made themselves at home quickly but would not go on the water for the first few weeks...thats when we lost 3 to the bobcat. After that, they began sleeping on the water through the summer months and all was well. Recently, when the tempertatures started dipping into the twenties at night, they started sleeping on the banks again and we lost one of our two drakes to a raccoon (the bobcat is no longer with us). Since then, the ducks have started staying the night on my neighbors pond...it is in the middle of a cow pasture and the predators can't ambush them as easily. I am currently building a loft in our barn that I hope to ecourage them to use on those extreme nights and also for nesting. It will be protected from climbing predators by a hot wire. If all goes well, they will come and go as they please and still have some protection.
All this doesn't answer your questions, I know, because I still don't know myself if I would do it differently. My remaining ducks seem to have matured and have learned to protect themselves, though we did suffer some losses. Sometimes I wish we had built a coop and pen, but then that would add to our daily time/work involved. We will see if our ducks continue to survive and increase.
The only advice I can give is if you go without pens etc., expect some losses. If you decide to provide some form of protection, prepare it BEFORE you get the ducks...and don't depend on chicken wire to provide that protection. You can make changes as you go, but, as in my case, once your ducks have complete freedom, it will be a bear trying to go to a cooped flock.
Either way, you will love muscovies, their comical personalities, and you will get a thrill every time those large birds fly overhead, then come eat from your hand.
EDIT to add: Our ducks love whole corn, we give them a handful every evening. They also like chicken scratch and a little chicken feed now and then.