If you find a way to actually damage 6 mil poly by crawling around on it, you must be wearing razor blades and spike tipped boots. 6 mil poly is almost harder to break through than tyvek.
Now, about the tyvek. DON'T! If you apply Tyvek to the underside of the joists, all that moist air will rise through the permeable tyvek, condensate on the bottoms of the joists and insulation, and be trapped behind the tyvek, which only allows VAPOR but not liquids to pass through.
Like Jay said, if you have water in your crawlspace, that is a problem in itself that should be corrected immediately. Common causes are: No gutters (You don't live in Texas, do you?) Landscaping "berms" around the outside of the foundation that direct water TOWARDS the foundation Downspouts (from gutters) that fail to direct water away from the foundation (or in some new houses I've seen; failure to connect the sewer line to certain random plumbing fixtures...ICK!!!!)
Get the water out of the foundation, lay down some poly, and go to a baseball game. Enjoy your summer!