I've used one of these for both of my 340 runs. They can't be beat as far as comfort goes, and this type won't go off if you accidentally (or intentionally) take a swim. You have to jerk the cord to inflate it. Obviously they don't have any storage built in, but a basic mesh fishing vest or equivalent can solve that issue.
http://www.basspro.com/Stearns-Multipurpose-InflataBelt&153;-Lite-Manual-Model-0...Im not sure if you plan to
wear the camelback or just have it nearby, but I would definitely try to rig a way to store your hydration
in or
on the boat.
1) It keeps a lower center of gravity.
2) You'll need at least two of the typical camelback pouches worth of water per leg (checkpoint) of the race, maybe more.
3) After 100 or so miles, you may not even feel like holding up your own weight, let alone an extra 2 liters of water

Hope this helps,
-Aaron