In rec.boats.paddle Walt wrote:

I'm not a whitewater boater, so take what I say with the right degree of
skepticism, but I would think that for a rescue line you want a bit of
stretch. For instance, climbing ropes always have a bit of stretch
(made out of nylon usually). The Yale and Samson lines in question are
intended for use as halyards, sheets, and control lines on a sailboat
and are intentionally made to have as little stretch as is possible.
I'm not sure that ultra low stretch is what you want in a throw line.

Boating whitewater, rescue lines are usually used to pull a boat off of
rocks or other obstacles. Stretch is wasted effort, and I suppose hides signs
of an impending failure.
Sometimes it is necessary to throw the rescue line out to someone or
something midstream for an extraction, so the throwbags get used in the
same manner.

For my throwable lines, I just use garden variety 3/8 inch 25 cents a
foot polypro - I don't see the need for ultra low stretch or ultra high
strength in a throwable.

Again, a difference in the whitewater environment. It might take significant
force to move a boat (or person!) pinned against a rock by the current.