Fishbird,isnt the EEC one tax bloc now.Otherwise no need for a boat.Just drive across a border every 180 days.
Europe is one big mess.
Denmark has 55%, Germany ( were i live ) 20%, Netherland 0%-50%, Belgium very high, Austria 25%-50%, Poland 16%, Andorra 0%, Spain 50%, France > 40%, Italy 13%, Norway/Sweden/Finland > 40%. And everyone has a different language too.
I dont think the registration of the boat has anything to do with your income taxes.
Even if you live permanently on a boat, you have to be a citizen of some country. But if you dont stay longer than 180 days this becomes unimportant anyway.