Yep, cottonmouths would be a concern. But consider this; there's feral muscovy populations in Fl and elsewhere in the south. The muscovy is a survivalist when it comes to predators (though not always). Part of the equation would be whether they can fly and prefer to perch or not. My black muscovy have never perched in a tree (though some do) and rarely get above 8 feet elevation. If they fly and perch, advantage nest box. I think this Amazon Forest duck would really take to any wooden cavity; meaning a wooden nesting box, see TreeDaddy post above. If they don't, an island may be the answer. My blk muskies love our 7' x 9' island. But then our biggest concern is the snapping turtle which I keep mostly shot out, we've no cottonmouth or copperhead. I plan to, in Aug 2025, to renovate the pond, push down the island, and replace with a 4' x 6' floating island. They poop everywhere but an occasional topping of straw should keep it somewhat sanitary. I'm planning on using 4" lite Charlette pvc to frame it. An easy access and exit is needed. Maybe reinforced half inch hardware cloth ramping down a few inches below the surface.
My ducks take to small water snakes like a dog to a squirrel. I've seen a colorful '?plaster of paris muscovy figurine' holding a snake, yet other than that I can't speak to the duck's survival in cottonmouth habitat. Hopefully, someone will share their experience with muscovy and cottonmouths.