text: Why do you consider it to be not a locality? It's unpopulated and a smal piece of the sea (so not worth for locality=sea). It's also a place that is mentioned in wikipedia (https://de.wikipedia.org/wiki/Unterwarnow) and at official webpages (e.g. http://www.rostock-port.de/livecam/webcam.html).
  Adding seamark:*=* might be useful, but IMHO is a different question?