Have you checked "General Sir Howard Douglas - An Essay on the Principles and Construction of Military Bridges and the Passage of Rivers - 1853" . It is filled with all kinds of diagrams and much on the Danube River Crossing.

Many of them can be seen in my article below.

I don't recall if there is much on the gunboats in the book. A copy of the books can be found at:

https://books.google.com/books?id=aZRPAAAAYAAJ&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q=Danube&f=false

The book is also in the Library of Congress.