Applying for subsequent certification of a birth on foreign seagoing vessels
Source: Serviceportal Rheinland-PfalzEach ship sails under a specific national flag and therefore legally belongs to the territory of that country while underway. If you have a child on a foreign ship, this is subject to the same rules as a birth in that country.
If one of the parents has German citizenship, your child will also have German citizenship. If only the father has German citizenship and the parents are not married, the father must first acknowledge paternity.
You can have your child entered in the German birth register, but you are not obliged to do so.
Your child can later apply for subsequent certification, i.e. a German birth certificate, themselves. Likewise, the wife, husband or descendants can later apply for a German birth certificate for the person born on the foreign ship.