Nylon stretches, IIRC, about 15-20% when wet. This isn't the whole story though.
A stretch rating refers to how much a fiber is stretched when it's at its breaking point. Being 2" nylon webbing has a breaking strength on the order of 5,000 lbs, you can imagine there isn't much stretch when it's weighted down with 40 pounds above ground, and maybe 2 or 3 underwater.
A tank cam strap, on the other hand, requires a large amount of force across the webbing.. this is why you get the cam strap wet before buckling the tank. Get it wet and stretch it a bit across the tank to strap it down tightly.
Polypro doesn't stretch nearly as much as nylon when wet, but you have to realize that at the small amount of force being put on the webbing underwater will mean there is effectively no stretch. The stretch is proportional to the amount of force on the fibers, so it's nothing to worry about.
Good nylon webbing is fairly soft, and I imagine it's worth it to spend the extra $5 to get the good seat belt quality stuff. There are no practical advantages to polypro, and it does have *far* less abrasian resistance than nylon.
