Ah, yes, the all-too-familiar tale of an upholstery job that turned into a shell-off!
Well, when my spruce-up job turned into a shell-off, I went with the marine grade plywood at ~$100 per sheet. There are plenty of threads here on the forums discussing the relative merits of marine grade vs non marine grade. Having gone through it, I am of the opinion that if I had to do it over again, I would use plain-Jane plywood and not incur the extra expense of the marine grade. I am told that the glue in both is equally water resistant, the wood in both is equally water absorbant, and the big advantage you get with marine grade is "higher quality" in that it has less voids, more layers, etc. Since I am not actually building a boat with it, I think this extra quality and expense is for naught.
I painted my new flooring on both sides and especially around the exposed grain of the edges with several coats of polyurathane, and my guess is that this will have a much bigger influence on the longevity of the plywood than whether it is marine grade or not. Just avoid anything that is "treated"--not what you want.