Interesting to just now read this on strapworks.com-
"Flat Nylon webbing is great for high abrasion applications. It is not recommended to use in or around water continuously because it absorbs water quickly, causing mildew and rot if not taken care of. Flat nylon will also stretch up to 2% of its length when the webbing is saturated."
"Polypropylene webbing is great for outdoor applications. It is available in 2 styles: Heavyweight, and Lightweight. Polypropylene has excellent UV protection, and it does not absorb water, giving it resistance to mildew and rot. It has low stretch, especially when wet. Polypropylene does not have a high abrasion resistance, so it is not recommended for use around rough edges."
"Sublimated Solid Color Polyester webbing is ideal for heavy loads with high abrasion. Polyester webbing has the same soft, smooth feel, and shiny appearence of Flat nylon. It also has the benefits of Polypropylene, low water absorption, and excellent mildew and rot resistance. "