First off, you are slightly confusing me. I don't think you have to worry about the walls of the crawl. BUT, do check w/the FHA Inspector to be sure. If the walls are 'leaking water', then you have a grading problem, and/or gutter/downspout problem, and/or water pooling problem, all of which (or in part) contribute to water in the crawl (or basement too.) I'm by no means an expert on FHA inspections but I think that to do the wall is WAY overkill.
RE: Covering the crawl's floor - Yes, use 6 mil plastic sheeting. You can buy it in the Home Centers, and will find it in the Painting Supplies Aisle. (Painters use it for make-shift curtains and covering.) The color doesn't matter. If, however, the plastic is exposed to sunlight, the UV rays will 'break it down'. So, wherever it's exposed to the sun, be SURE to protect it.
You needn't tape the seam together (as long as the FHA Inspector says you can 'overlap' the seams.) Just find out what they recommend as the MINIMUM amount of overlap. Ask them if you can use gravel or Pressure Treated lumber to 'hold' the seams down. Use the same type of material along the edges too to keep the plastic down. Use anything that won't rot. Just DON'T go attaching any plastic to the bottom of the floor joists or anything.
One last thought - Be SURE to get SPECIFICS from the FHA Inspector as to what you should do. Ask if the plastic in the Paint Aisle is OK (if it's 6 mil thick.) If not, ask them where to get it and what it's SPECIFICALLY called. Ask them if overlapping XYZ inches is OK if you use gravel or PT wood to hold the seams down, (there as well as along the edges.) Again, if not, what should you use AND do. The reason this is SOOOOO important is because if you DON'T do it 'right', they'll have to come back yet another time. ALL this type of 'stuff' does is prolong the sale process. They are tough folks.