Danzig is somewhat promising, although that is a pretty big area, as Danzig (modern day Gdansk, Poland) was one of the largest and most important Northern European ports for centuries.

I have family from that area (Bendig, Bendicki, Brooge, Grabowski) from the nearby towns of Langenau, Sommerau and Freystadt. The Family History Center has extensive microfilm records from the Evangelical churches and the Catholic Churches from West Prussia which diligently record baptisms, marriages and deaths from the late-1600s/early-1700s through the late 1800s. I just ordered the records for Langenau and will remember to look for Louis Ruh when they come in.

Also - since Louis Ruh arrived in the U.S. prior to the 1890s, there is a good chance that he arrived at Castle Garden in New York City (the forerunner of Ellis Island). www.castlegarden.org has a reasonable search engine. I put in Ruh from 1820-1885, and it returned 150+ names. So - check that out to see if any of them seem promising.