Tom Rolt (who was an inspiration behind saving and using Britain's canals for leisure) originally had a petrol engine (ex-Model T Ford!) in his narrowboat Cressy in 1936. And he was not alone in using such an engine, although they were never common - horsepower generally gave way to diesel. Some shorter narrowboats are powered by a petrol outboard motor because a diesel engine would take up too much space. Whilst a 'marine' diesel engine is obviously ideal, there are many engines of the road vehicle type; my brother's narrowboat uses a British Leyland engine of the type fitted to a light van.
Yes, the 'pop pop' of a Lister (or similar) diesel is quite distinct and certainly reminiscent of the African Queen.
A Btitish 'narrowboat' is so called (and built) so as to be able to cruise on the narrowest canals, a nominal 7'0" wide, with the boats a couple of inches less. The canals were made narrow at the locks, bridges etc to save cost and water, but obviously had to be wide enough in most open stretches to allow vessels to pass. By no means all of British canals are 'narrow'; many are 'broad' - wide enough to take quite large barges and similar craft, although of course such commercial traffic has largely declined.
Yes, the usual narrowboat drive is by a directly-driven prop shaft with a simple speed/reverse lever control. Simple but effective.
