It seems a bit strange that there appears to be no way to extend it, especially when I think a non-EU boat can stay for 18 months.
There is much confusion over the 18month rule
for the boat and the 3 month rule
for the crew. Basically , a non EU boat can remain in the EU for 18 months. Thats the boat ONLY not the crew, The idea behind the legislation is to allow you to leave the boat and return to it in the next season and continue your cruise
. It does not give the crew of such boat any extra time in the Schengen Area of the EU.
non EU crew are merely tourists, and abide by tourist visa rules, for schnegen countries this is 90 days in 180. This is a significnt reduction on crew freedom, as previously each country would give you between 3 and 6 months so in theory you could stay in the EU for ever. Thats gone. You now regard the Schengen area as effectivly one country for visa purposes.
countries outside that area, namely UK and Ireland, will individually
give you betwwen 3 to 6 months on a tourist visa. This is not factored as "Schengen time". Note that you have to exit the Schengen countries for the remaining 3 months.
In time Ireland certainly and the UK possibly will join the Schengen, Ireland is only there because of the UK position as it doesnt want to jepordise its common transit area ( ie no passports needed) with the UK. Uk e-borders is in the process of removing that anyway, so I suspect Ireland will be in Schengen before the end of next year.
Note that all these rules apply to Schengen Visa Wavier ( ie US citizens) holders as well.
Long stay visas are country specific. These are usually work or study ones, though there are a number of non-work ones for austrialian and new zelanders etc. However they only give you rights to stay in that one
country. Once you travel outside that country the Schengen rules apply.
Also be careful as being resident for 182 days or more in most EU countries makes you a tax resident and unless you meet the requirements for "Transfer of Residence" then any boat ( or car) will be subject to VAT. This can happen much sooner then 182 days, for example taking paid work, or certain visa categories can effectively make you resident. This needs careful inspection