Have you considered a small sailing dinghy? Something like a GP14 for instance, GP for General Purpose 14 foot. Designed to be rowed, sailed and propelled by outboard motor. Will sleep two onboard, size dependant, and are capable boats which can be sailed solo or carry four, though I've seen more. Or a Wayfarer or Wanderer which are regarded as quality cruising dinghies, that said you could probably go to any sailing club in the land and pick up an old unloved Mirror for next to nowt. A surprisingly capable boat the Mirror and is car topable like a canoe.
I suggest you take a look at www.dca.uk.com for a few more details about dinghy cruising and camping.
I say this as someone who has paddled canoes for many years and would have readily agreed that a canoe is the perfect bushcraft boat, but it isn't really suited to open water and the sea. It can be improved with the addition of an outrigger in the manner of Proa's etc to make a very seaworthy vessel, but the modification might put many off. I'm now thinking that a dinghy is the way forward. They address many of the issues you raise with many having lockable waterproof chambers and sleeping onboard under a boom tent.
Food for thought, I hope.
Very interesting. I think you are raising more interesting options.
I think a mirror is a bit small, but the Wanderer looks interesting, but no build your own.
Something like this on the other hand is very interesting. julie-skiff