While I have not needed to use it to date, I have a RESQME zip-tied to the pocket zipper of my rescue PFD.
Pete -
We did an "in the water" job a few months ago, a drunk into the canal. I had a standard push & snap punch in my hand, which I promptly dropped and lost the moment I got to the truck (the driver was actually in an OK space for air, we just needed to get the window out to make it easier to get to the "B" post. Since then, I've put a retractable lanyard on a punch for the in-water rig. We've been doing a lot of in-water rescue stuff lately, which has been really informative and useful, especially if you manage to get to someone who's alive but submerged - which is possible in some limited situations. The transition point - you have a car, there's SOME air in it (a pocket) and now you've got to leave the air pocket and surface, with a patient who might be broken. Just putting a drill together for this scenario is dangerous, actually doing it is even more risky.