There's no legal way---or any way, to my mind---to penetrate a dense marsh should you know where the duck is hiding, certainly not by a set of oars. It's illegal to chase down a cripple under power.

My Labs have picked up broken-wings 400m from where they went down, almost always from places I hadn't imagined. An energetic broken-wing duck or goose can move faster than anyone in a marsh or field of corn.

As for picking up after a flurry of shooting, my Lab last week brought in five dead from the water and two cripples in blind retrieves from a nearby cornfield in less than seven or eight minutes. Try that in a boat.