I encountered Ssiwasch in the novel Der Zug war pünktlich and it is mentioned in a few other places on the internet. It clearly refers to the Syvash which Wikipedia claims is called Sywasch in German, so presumably Ssiwasch is an older spelling which has been abandoned.
How did the word come to have such a strange spelling? Is it something to do with translating Ukrainian words into German, or is it some sort of misprint which was perpetuated through time? Are there any other German words that begin with two esses?
In the meantime I have found a source which seems to apply that з -> s and с -> ss was one way of transliterating Russian words, and it makes sense. (Although amusingly, it does not recommend transliterating сс as "ssss", because it "doch wohl unthunlich erscheint.") This is from the introduction of "Der Russische Feldzug nach Chiwa" by H. Stumm in Google books. The book actually uses only a single s for a Russian c at the start of a word, but it seems to imply that some people used a double s for a Russian c everywhere in a word.