Hi kenomac, it might be better to PM me but I'll give you the basics.
For EU citizens we have an EU firearms passport. That helps ease the issue. But we are still subject to individual country rules.
For example to transport my target stuff across boundaries I generally need an invite from a foreign club plus in some cases a temporary license
from that countries local police. Further for the person that said " carry them in checked luggage " nothing of the sort is really like that. Usually to fly I need to be an airport
4-6 hours before and go throw an extraordinary rigmarole with customs
security etc. airlines then all have different requirements as to what you can carry. Its not simple. ( especially transiting on point to point airlines in countries that ban certain of my firearms )
In your case if you are trying to bring in a firearm by air from outside the EU. Forget it basically.
If you intend purchasing
it in the EU , not being a resident
Forget that too.
If you are bringing the Mossberg on with the boat , as has been pointed out , you are not importing the boat nor the firearm so don't read stuff about 'importing firearms ' this is a trap Gord may fell into a while back.
What you will find is that on arrival on almost all med countries, is that you will be required to declare the firearm and ammunition.
What then happens is very much a function of the customs / police you meet. Some will have a very very negative attitude and will apply the letter of the law. This typically means the fire arm will be detained , transported to a secure lock up facility and you can collect it from that office on your departure. All this will incur charges, some quote steep.
In other cases if you are a fairly large vessel with a proper steel
lockable gun safe , they may customs seal it till you depart.
Note that if the firearm is of a type that is illegal or even heavily restricted, it could simply be seized and you could be prosecuted. This includes tactical shotguns , semi autos, handguns, firearms with large calibre magazines, silencers, night sights , etc ( even though they may be legal
for that countries citizens to own )
What you will also risk , is abject pandemonium , many Med countries have few customs and immigration people in leisure ports
anymore. When you declare a firearm all hell can break loose. I know of one instance where the marina asked the yacht to leave immediately
All in all , its not worth it. And in the med there's simply no point to carry a firearm.
Leave them at home is the best advice
If you have specific question ask away