I don't tether anything to my canoe except small items like the small stuff sack that carries my camera and phone. My philosophy is to pack in as few big, bulky items as possible. Emptying a canoe is much quicker if you don't have to untie your luggage first. The few big, bulky stuff sacks are easily spotted and retrieved by our paddling buddies.

To tether or not to tether... without paddling buddies I may decide to tether everything. Like AndrewC said, if you tether get it tight so it won't snag.

No experience with kayaking (well, tried it a few times - great fun!) but I gather storage space is rather limited - not only in volume but also in the shape that can be stuffed in there.

Waterproof bags... are not really that nescesarry, at least not all over all at once unless you have the money for it. Decent waterproof bags are rather expensive. Get something good waterproof bags/containers for your electronics and your spare clothes, sleeping bag and perhaps some other things you'd really like to stay really dry in a survival scenario. The rest of the gear (assuming you'll be packing camping gear) can tolerate a bit of water without being a treath to your existence. If you line ordinary bags with some heavy-duty garbage bags you've just made yourself some pretty water resistant bags that will float. That'll do it...