Though I'm not 100% sure, but I think writing on the back reads:
U-Boote im Hafen von Swinemünde = Submarines in the harbour of Swinemünde and
Hafen von Swinemünde = Harbour of Swinemünde
More info about Swinemünde on Wikipedia
Cheers, M. Baecker
Thank you very much for your kind help!