In addition to the information provided by KlausWa I would suggest contacting the "Heimatkreis" for Stolp to see if they have any additional information. http://www.stolp.de/arbeitskreis.html
German Navy records I believe survived the war relatively intact, so there should be some information available. You need to contact the Deutsche Dienstelle and submit a tracing request. Ignore the part about payment for now - they will send a response in about a year which should contain info about his service, at which time you can opt to order copies of the actual records. http://www.dd-wast.de/frame_e.htm
In regards to "border" it may have something to do with this:http://de.wikipedia.org/wiki/Marine-Brigade_Ehrhardthttp://de.wikipedia.org/wiki/Grenzschutz_Ost
Incidentally, on my father's side we lost three family members on the Gustloff.