Score the fat side of the chilled breasts with a sharp knife into a 1/2" hatch being careful not to pierce the meat. Drizzle with a little olive oil and liberally season the breasts on both sides.
Place in a vacuum-sealed bag or a Ziplock, and use the water displacement method to remove the air and seal the bag.
Cook in the water bath for 1 1/2 to 3 hours.
Remove the bag from the water bath and place on the counter to cool for about 5 minutes before submerging in an ice bath to chill before searing. You can also chill, refrigerate and sear later.
Remove the breasts from the bag and pat dry with paper towels.
To sear: Preheat a heavy-bottom skillet over medium-high heat then place the duck breasts skin-side down for about 5 minutes to crisp the skin. Turn the breasts over and remove the skillet from the heat. Continue to cook on the flesh side for 30 seconds to warm through.
Transfer to a cutting board and rest for 5 minutes before slicing and serving with a sprinkle of coarse salt on top.
Duck breasts can be cooked sous vide ahead of time, chilled and seared right before you are ready to serve.
The recipe is for two duck breasts but can easily be scaled up.