For a successful rough water rescue, to begin with, hopefully both boats will have perimeter lines and the rescuer will be wearing a spray skirt. The person to be rescued will have a firm grip on their boat, either by the cockpit or a perimeter line. Both people will have at least a little experience with an assisted rescue, although if time is not a factor the rescuer should be able to talk the person being rescued through the process.
I’ve found that often the hardest thing in a rough water rescue is getting hold of the other person’s boat. . Both boats are bobbing around and the rescuee’s boat is being somewhat anchored by the person in the water while the rescuer’s boat is probably being blown by the wind. May take a bit of trying. Start where your boat is being blown toward the other boat.
Once you have hold of the boat work your way toward the bow of the rescuee’s boat. The person in the water can work their way toward the stern of their boat to aid in getting their bow up onto your front deck with their boat perpendicular to yours. Once you have a firm grip on their boat you should be stabile enough to not risk capsizing your own boat. From here you use the standard technique of hauling their boat onto your front deck and with the rescuee’s help invert their boat and drain the water out. Hopefully your boat does not have it’s front deck junked up with delicate gear and the person in the water still has a firm grip on their boat. It is probably a good idea the the person in the water not be directly in line with either the bow or stern to avoid being hit in the head as the boat moves with the waves.
At this point maneuver the boats into a bow to stern configuration. The person in the water can then work their way to the cockpit in preparation of getting back into their boat by either climbing up onto the rear deck or using the heel hook technique.
When the person in the water attempts this, you should have a death grip on their cockpit rim to hold and stabilize their boat. Partially leaning across their cockpit makes this easy. In all my experience, I’ve ever heard of anyone injuring their shoulder while doing this and can’t imagine how else you would do this without putting more strain on your arms or shoulders. When in this position you are very stable and can’t capsize unless you lose your grip. It is essential that you have a spray skirt on as your cockpit rim will often be under water in this position. There’s probably more risk of injury hauling their boat onto your front deck to drain it, although there aren’t many options. Let the waves assist you.
Once the other person is back in their boat keep hold of their boat until they have pumped out any residual water, put their spray skirt back on, and reorient themselves. If necessary, they can put their spray skirt partially on while leaving just a small opening for a pump. It only takes a small amount of water sloshing about in most kayaks to render them very unstable.
I’ve left out some of the finer points of an assisted rescue which I’m sure you already know.
Never, never, never leave a person alone in the water to retrieve a loose boat. If there are more than two of you, have a third person retrieve the boat. If just the two of you, hopefully you can tow the person back to shore. You’d be surprised how fast a loose boat can be blown away and how far it can go. In rough water, it can be extremely difficult to locate a bobbing head in rough water.