Generally speaking, the answer to the question which driving licence you require always depends on the destination where you want to go boating. Rules differ widely not only from one country to another but also in different regions and on different waterways within one country.
Regulations in Germany
Sailing boats without an engine are not subject to a nationwide boating licence requirement in Germany. There are, however, boating licence obligations regulated by federal county law that may apply outside of federal waterways or on inland lakes. For example on waterways in Berlin you require a sailing licence from 3 m² sailing area upwards and in Saxony from 6 m² upwards. On Lake Constance a Lake Constance boatmasters licence D is required for a sail area of 12 m² or more. It is also up to the owners of a waterway or lake (e.g. city administration) to issue individual boating licence requirements. In addition, many commercial boat hire companies require the presentation of a boating licence as proof of your boating competence.
Motorboats with an engine of up to 15 hp (11.03 KW) do not require a boating licence on federal inland and coastal waterways in Germany if the boat is no longer than 15 meters and the skipper is at least 16 years old. On coastal waters small boats with a less than 5 hp are exempted from the 16-years age limit. Parental responsibilty, of course, applies. For all leisure boats with an engine of more than 15 PS a boating licence for Leisure craft is required on all federal waterways in Germany.
Rhine River & Lake Constance
The River Rhine is exempted from the 15-hp-rule due to the high commercial traffic. On the Rhine, driving licences for recreational craft with an effective power of more than 3.68 kW (5 hp) are mandatory. And on Lake Constance special licence requirements apply due to the cross-border agreements with Austria and Switzerland.
Since 2005, skippers of recreational boats are required to prove their qualification by holding the corresponding radio operation certificate in accordance with the radio equipment of the yacht. The new radio operation certificates SRC (Short Range Certificate) and LRC (Long Range Certificate) are only required for skippers if a corresponding GMDSS-capable radio equipment is on board. Non-compliance is a punishable administrative offence with a fine of € 150.
Regulations Outside Germany
Each country has its own driving licences and regulations for navigating sailing as well as motorboats boats. However, in the territorial waters of other states, the skipper requires at least the boating licence required in his home coastal waters. While the port authorities within Europe usually mutually recognize the boating licences for the coastal area, you have to check the validity of your licences outside Europe individually with the local authorities.
No boating licence is officially required for sailing on the high seas once you are outside territorial waters. Independent of the legal regulations, however, charterers of sailing yachts or insurance companies often require proof of additional knowledge of the skipper and sometimes also of the coskipper. Such additional knowledge can be provided, for example, by voluntary higher licences for the corresponding cruising area or by nautical mileage certificates.