On river trips that involve any type of whitewater I would include provisions to add some supplemental flotation to your boats.
Tandem canoes usually have enough room in the stems for a pair of short tandem end bags, one behind the stern paddler's seat and the other in front of the bow paddler's legs. These will help the boat float a bit higher in the event of an upset. Bags and gear can be restrained under accessory cord made of nylon or some other synthetic material about 3 mm thick. A strap of nylon, polypropylene, or polyester webbing about 1" wide attached at the stem of the canoe on one end, and to an anchor on the floor of the hull at center on the other end will keep the bags in the hull below the gunwale line, and pushed up in the stems of the boat where you want them.
Accessory cord lacing that runs transversely back and forth just beneath or at the gunwale line can also be used to secure packs and gear under. In this case, you want to be able to easily lace and unlace the cordage.
I would not attach anything else to the gunwales. In the case of a capsize, you don't want heavy gear attached at the gunwales hanging down into the water to get snagged. Gear attached to points at the gunwales also make boat-over-boat rescues difficult or impossible. Granted, boat-over-boat rescues may not be feasible for canoes with a lot of gear lashed in, but having a bunch of crap hanging from the gunwales can make getting the stem of a swamped boat up onto the bank so that it can be inverted and drained difficult.
On Royalex boats to which I wanted to add flotation bag lacing I have generally just drilled holes through the hull spaced about 4-6 inches apart and threaded the cord through. This works very well but if the cord needs to be repeatedly loosened or removed it becomes laborious. You can buy lacing kits that have nylon "inchworms" to thread cord through, or you can drill a pair of holes about 3/4" apart and run a short loop of accessory cord through each pair to function like an inchworm.
Small nylon "P clips" of the type sold by Radio Shack to secure coaxial cable also work well and are much faster to lace cord through than holes in the hull. These can be secured using aluminum pop rivets just under the gunwales of an aluminum boat, or to the undersides of synthetic gunwales on a Royalex boat using stainless steel sheet metal screws.
I have always used a vinyl adhesive such as Vynabond to secure D rings based on vinyl patches to the hulls of Royalex boats. I have not attached D ring patches to aluminum hulls. I doubt that Vynabond would work. Contact cement usually does not provide enough grip if there is any real tension on the D ring. West System's G Flex epoxy (which can be purchased in smallish quantities for about $25) is said to work very well on aluminum and it will definitely bond vinyl D rings. If you use an epoxy it will not provide an immediate tack bond, however, and you will need to secure the patch in position with something like duct tape, and preferably gently weight the patch to avoid air voids while the epoxy cures.
Pull-Up Strap Handle Kit
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