If nobody is there and it is public, the spot it yours. I would pickup their decoys and set them to the side and hunt it. I have been on both ends of this before. If someone is there when i get back i'll pack my crap and tell them good luck.
There is no reason to be rude in marsh, well most of time anyway. I do treat other peoples gear the same as i would like mine to be treated though
Public ground is just that public, and as such just because somebody leaves their gear out doesn't mean the spot is there's. On a side note when i'm hunting now days i dont' take a lunch. Once the decoys are out we are there till they are picked up.