One of the biggest problems, is that "The Law" be it National or International Law, is subject to "interpretation" first by the National Government enforcement agencies, and then by local regulatory officers. Then, you also have the equivelent of Bye-Laws which can be different for every port visited. We have some anomolies here in the UK in just the same manner.

Just one example: Who is the law enforcement agency for UK waters, such as a River Estauary?
1. The Police
2. The River Bailiff
3. The Coastguard
4. The Harbour Commissioner
5. The Environment Agency
6. The Local Council
7. The County Council... etc.

It can get complicated, as differing bodies have differing responsibilities. Now multiply that by the number of Countries you want to visit in a cruise, and Customs, Immigration, Heritage Depts ( if you want to dive and the like).

One of the points/raison d'etre for the EU was to harmonise all this legislation into manageable proportions. EG. Boat must be certified to regulations pertinent to Country of Origin, in UK that being MCA Rules (which would probably need to be Code 0 for Greece, even if sailing in local waters), and that would be recognised by Greek authorities. But... it doesn't seem to work like that. Safest, is to be Greek or have a Greek partner that knows the wrinkles.

Sorry, semi-rant and wandering a bit off topic.