For paddling in anything except the most sheltered/populated conditions or area, I think a personal locator beacon with GPS is the single most important thing one could possibly carry.
With the push of a single button, your beacon will notify Langley Airforce Base of your precise location and that you need an immediate rescue (the GPS locator elimintes the "search"). They will then be sure that the appropriate rescue team is activated pronto. The technology is extremely reliable and robust, and has been used successfully for decades for troubled ships and downed planes.
You can get one now from McMurdo for $250 (or less) which ways only 5 ounces and is absurdly compact. It is completely waterproof; however, it does not float, so be sure to keep it tethered. There are others that do float.
I can think of no good reason why one would not carry one of these at all times. You've already invested big bucks in the rest of your gear...how much is your life worth? By carrying it on your person, you could still summon a rescue if you became separated from your boat.
Seriously...just buy one.