For immigration, you must arrive first at a recognised port of entry in each EC country you visit. You will be allowed a maximum stay of 6 months in each country (possibly extendable). Unlike USA, you do not need to recheck in at every port.
Why why is this wrong information constantly put up on US boards
firstly ignore anything that is based on UK rules, They are outside Schengen
(a) you will
only be given 90 days out of any 180 tourist visa to visit all the schengen countries, ie treat all the schengen countries as one country. You
Will not be given 6 months in every country, this is a pre-schengen rule.
(b) US tourists do not need to apply in advance for a visa, you are automatically given the visa as a passport stamp when you arrive at the first point of entry into a schengen areas country
However the stay rules are the same for US as any other touristIts simply a reciprocial arrangement for the US Visa wavier scheme, non needing a visa does not mean that the schengen rules dont apply, They DO. again this is a source of much confusion amongst US boards
(c) technically you do not need to recheck in at a port in a schengen area when travelling from another port in a schengen area, however many countries have specific rules for yachts ( ie Greece, portugal, etc) which
do require you to check in , Northern Schengen countries tend to follow the no check rules whereas southern ones tend to require you to recheck. Note if travelling from the UK or Ireland back into the Schengen area you have to reclear in. ( You also have to clear out of the schengen area as well. and some countries like greece have specific rules for yachts, inc transist logs etc
(d) Use of the Q flag is declining in the EU. So generally its not neccessary, though I know Greece , portugal and sometime spain are hot about it. Generally to clear in you will arrive at a marina in a port of entry, go to the marina office and begin the clearance process there. Very rarely do they come out to the boat. ( you arnt a 30,000 ton tanker!).
In my experience especially of spain/portugal, the marinas will require boat insurance and particular third party insurance and public liability insurance before allowing you to berth. This was less so in france and not at all the case in the UK. Note this isnt a government requirement, merely a marina one
(e) the UK will give a US visitor 6 months entry for the UK alone.
Longer visas can be obtained for the Schengen Countries (which don't include UK!) for certain (narrow) categories of visitors - ask at one of the Schengen country offices in the US.
Not TrueThere is NO extended Schengen Visa, this is another myth perpetuated on US Boards. Some EU ( note EU) countries have extended visas , but this
only apply to that country They do not extend the 90 days out of 180 rule. also these country specific visas are generally non-tourist and generally make you a tax resident and hence you can loose the 18 month VAT excemption. Also These entended visas are for cruisers, almost impossible to get as they require residence in the visa country.