I don't quite understand this phrase
The Rib trailing off the back of the Naga
Does it mean "The Rib trailing behind the back of the Naga"?
Judging by the picture, I suspect that the "Rib" is name of the boat in the picture, and the "Naga" is the name of the boat that's towing it (and where the picture is being taken from).
In this context, the phrase means that the boat in question - the Rib - is attached to the thing it's "trailing off the back" of: namely, the Naga.
"behind the back of", by contrast, does not imply that the boat is attached.