Walt wrote:

Talk to the climbers. They're even more fanatical about their lines
than we sailors. Climbing line is always a little stretchy, and the
stretchyness is not considered a sign of impending failure. That said,
a throwable rescue line doesn't need to be as stretchy as climbing
safety line, and probably shouldn't feel stretchy at all.

Different tools for different situations. A climber's line stretches to
absorb energy. After a long fall, the line is removed from service. There
isn't the option to take it out of service mid-fall. A line to a wrapped
boat can be a hazard if it breaks, throwing hardware at high velocity. If
it shows signs of hitting its limit, the line can be replaced or assisted
with a second line.

3/8" polypro line has a tensile strength of almost 3000 pounds. You're
going to heave 3000 pounds? By hand? Would your kayak stand up to 3000
pounds of force? Would your potential rescuee?

Me personally? No. 10 people using a pulley-based mechanical advantage
system? Yes. Might that much force be required for a fully submerged,
heavily loaded raft?
http://www.cacreeks.com/photos/z-blsm5.jpg
I think so.

Actually, I'd be curious as to what serious kayakers are using in the
throwable bag.

Here's what one company sells:
http://www.nrsweb.com/shop/product_list.asp?deptid=1039

I have one of the 1/4 inch polypro lines, which I concluded recently is NOT
suitable for pulling things off of rocks.