Their original business midel is not permitted in Germany: private people are not allowed to do commercial person transports.
Use taxis instead. The drivers are educated, cars are licensed (number in the rear window) and the driven distance is metered and priced. Nearly all taxis accept credit cards - ask for when booking one. I am sure the cruise team knows the numbers for all ports. Tip. always ask for a receipt (Quittung). If problems occur then you have the address and phone number of the company and also the license number (not license plate).
Other option are chauffeur services.
In some regions you can call taxis or chauffeur services with Uber but I am not sure that this works everywhere.
Also be careful because Uber and some other companies are known for supporting illegal drivers - well knowing that they do. In Berlin after a check by authorities 1,700 cars had to be deleted out of their databases (source) - in Berlin only.