John, I read this with dread having just completed a long trip where this type of scenario was our most identifiable risk and certainly my biggest fear.
Our trip took us offshore ranging from 85km to 130km, with big crossings every day. We paddled boats that moved faster than traditiona sea kayaks, in trade wind conditions mostly, using small sails which lifted the heavily laden boats and gave us cruising speeds of 9-10kmh. This is fast enough to put a kilometer between paddlers in as little as 5 minutes in the event that one capsized or encountered a problem.
We put in place protocols to deal with this. In the event of a capsize and failed roll, self rescue, a whistle blast, no longer than a minute after the event. Next step was a flare. Next step, no longer than 5 minutes was the VHF, which we were all carrying. Final step, no longer than 15 minutes was to set off an EPIRB, all either on our PFD or strapped to our bodies.
All three of us have years of rough water guiding and instructing under our belts so we are trained to watch out for our group. In the hurly burly of rough water paddling, where you tend to become self absorbed and lose all sense of time you're kidding yourself if you think you can just do this, it takes a lot of practice.
On the two occasions where one of us needed help the other two noticed the problem within a minute, that's how closely we paid attention to one another, and how small a group spread we were able to maintain. This was part of our pre-agreed plan for each days paddling, reiterated each morning before we cast off.
In keeping with the theory that most incidents occur before you leave the beach, we had discussed the scenario and felt we would have dealt with it whatever the catastrophe.
Our distance from help was obviously a factor in this level of preparation, but we shouldn't forget that there are days where even a couple of KM offshore might as well be the moon.