I think this depends what you're used to. Your 747-captain friend isn't used to flying single-engined aircraft over water, and why should he be? Other people do it all the time.
The school I rented from in Phoenix won't let visiting renters take their aircraft cross-country until they've had a "mountain flying checkout", which includes discussions of density altitude, mountain meteorology, need for higher throttle settings at higher altitudes, etc - it was very useful.
After planning the flight, I set off with the instructor. About 1/2 hour into the flight, I mentioned that there wasn't really anywhere to put the plane down if the engine quit. He just shrugged, and said "I'd head towards that road over there, we could probably glide to within a couple of miles from the road, hopefully find somewhere flat enough that we'd be able to get out of the plane and walk to the road for help." Of course, he flies in mountains all the time, and was far more complacent about it than I was. But after a couple of weeks, flying over mountains with nowhere to land didn't bother me either. I did a 200-mile cross country from Phoenix to Grand Canyon, and once I got out of the Phoenix area I only saw 3 good landing sites for the next 150 miles or so (2 airfields and the Interstate) - but if I'd let that bother me, there'd have been very few interesting places I could have flown to.
FFF. .---------