An effective technique for carrying a swimmer is to clip two kayaks together with a pair of contact-tow straps. The rafted-double configuration works amazingly well as a platform for carrying someone (see "The Rafted Double," SK December 2000). In trials, we were able to carry a swimmer very effectively for a considerable distance. With the load distributed over two kayaks, neither is badly out of trim, you nearly have the speed and stability of a tandem boat, and both paddlers are free to paddle and brace.

Contact-tow straps
Although you won't find ready-made contact-tow straps at your local kayak shop, they are easy to make. The contact-tow straps I use are based on one I saw used in a towing class at the Great Lakes Sea Kayak symposium. A single contact-tow strap consists of two 3' lengths of flat webbing, with a carabiner at each end, and a quick-release buckle in the middle.
I prefer buckles that you can adjust on both sides, so the strap can be lengthened or shortened from either side of the buckle. Most buckles adjust only on one side, so you have to inspect the buckles carefully before purchasing. If an attachment point for the webbing has two bars, it is adjustable. If it has a single bar it is not. The instructor that originally showed me the tow-strap equipped with a buckle used a 3/4" buckle. I thought it too small to release quickly with cold hands or gloves.
I use 11/2" webbing and buckles.
The first carabiners I bought for this project had the little notch in the clip end. The notch constantly caught on the deck lines, so I bought carabiners that had a flared end instead of a notch to connect to the gate.
Heat-seal the cut ends of the webbing with a match or lighter. Sew a loop to fit the carabiner on one end of each piece of webbing. If you don't have access to a sewing machine, see if your local shoe repair shop can do the job for you. Thread the other end of the webbing through the buckle. To keep the webbing from slipping off the buckle and to provide a more positive grip, tie an overhand knot or sew a double fold in the end.
When I practiced with contact-tow straps, I worried about having one fall into the water as I was hooking it to or unhooking it from the deck lines. To keep it from sinking, I added a float to each end. I cut out some small blocks of foam, trimming them to make them as small as possible but still able to float the carabiner. I cut small holes in the foam blocks and slipped them over the webbing.
The contact-tow strap has a number of advantages over tying or clipping kayaks together. It can be pulled tight to draw the kayaks firmly together and it can be quickly unbuckled, allowing the boats to separate. The versatility of this allows it to be used for many applications, including, of course, as a short tether for a contact tow.