The closest thing I can think of to an amphibious excavator would be a dozer with LGP wide tracks for swamp use. While excavating catfish ponds, my Cat D4D LGP worked without sinking in oozing mud that could barely support a man's walking weight. Drain the pond as best you can and call in an LGP dozer before the water starts to seep back in. I have seen a drag line sitting on barge being used to widen a river's course, but that setup would probably cost far more than its results are worth.