>>2324838Research it, boat building is the last rather unregulated thing you can do in Germany. As long as the boat is less than 20 metres long and has less than 5PS it will be easy to register and you don't even need a licence to drive it.
Source of the pic:
http://matthiaswestphal.tumblr.com/archiveCombine this with a DIY powerwall made out of 18650s and you can be totally off the grid.