Correct, the more your propeller is working in "open water", the better it is.
One exception is the jet drive though, where the propeller (actually impeller) is so efficient because it hardly has any blade tip losses. I wonder when or if they'll come up with an electrically driven rimdrive waterjet propulsion. It seems like the best of both worlds to me. No shaft going into the waterjet, just a cable on the outside.
Maybe I should take a patent on that...
Oops, too late, it's already common knowledge now.
Bruno