What regulations apply to radio operations in International waters? How far offshore does one need to be? Does the answer change between registered commercial vessels and home-made rafts or canoes?
For registered commercial vessels, you have to have permission from the country of the ship's registry AND permission from the operator of the ship AND permission from the captain. Source.
For being in your own home-made craft, I imagine you just need your license.
Note that everything changes when you're in territorial waters, and I'm not sure how you know (I'd guess maritime charts would indicate where they are).