-- Last Updated: Feb-20-13 12:12 PM EST --
Pblanc covered attaching D-rings to the floor of a Royalex boat. In my Royalex boats, which are solo models, I've installed a set of four D-Rings ahead of the seat and behind it. Each set works great for tying packs to the floor. For a tandem canoe, I'd install about six or eight rings (three or four in a row on each edge of the floor) in the center part of the boat. D-rings on the floor will be far more effective at holding your gear down than tying ropes to anchor points on the gunwales, because the ropes will be "pulling down" and cinching everything tightly to the floor, rather than simply overlapping the top of your gear. Attachment points on the gunwales might be handy at times, but on a Royalex boat, I wouldn't install them for any purpose except to make a float-bag cage, and even then you are better off just drilling the hull as Pblanc describes.
I'll add one more note about gear that's lashed securely to the floor. Not only will it not get snagged on anything or get in the way during recovery of a swamped boat, it actually adds to the flotation of the boat. Ever see an unsecured gear pack get away from a swamped boat? Did you see how high it floats? It definitely helps if you can make your packs "one with the boat" by securely lashing them to the floor. Of course, this assumes your packs are either waterproof or properly lined with well-sealed plastic bags (NOT garbage bags - garbage bags are far too flimsy). By the way, there's no need to be able to quickly remove gear from your boat for recovery from a capsize. Since lashed-in gear aids flotation, it's actually easier to get your swamped boat to the shallows with the gear in than not.
For the aluminum boat, I think the ideal thing would be to get a welder to install aluminum patches, with D-rings, onto the floor (I've never heard of someone doing that, but of course it can be done). I also see no problem with bolting such patches to the floor (you already have rivets poking through, so tiny, round-head bolts would not make things any worse), but make sure you seal the contact surface between hull and patch. In the first case, you would need to make (or have someone make) custom patches, so this takes someone's talent and time (which may cost extra). In the second case, you could use steel patches, and can probably find them ready-made with D-rings attached, but they will rust (that wouldn't bother me - they'd still last virtually forever). Here's a case where D-rings bolted to the gunwales might be okay, but I'd keep the number to a minimum and avoid drilling holes any bigger than you have to. However, if you are only attaching ropes at gunwale height, the thwarts are already going to give you similar tie-down ability, so maybe skip the gunwale attachment points altogether. I guess another option would be to install them partway down the sides, proving better direction of pull for your anchor ropes but still not requiring waterproofing where you drill holes to attach the anchor plates.