Sure, you can get different types turbulence over land, and you can hit different weather fronts over water, but in reality flying at 40,000 ft it doesn't really matter what you are flying over as you are above most of the weather.
If you are concerned about being 'far away' from civilization while flying over water, most 2 engine jets fly within a certain set range of the nearest landing areas, so it really isn't that much different then flying over some of the desolate land areas of North America and Asia. 4 engine jets can go out farther, but they are still required to stay with certain ranges.
Like Penguin said at 40,000 feet doesn't much matter. I believe two engine jets must be within 180 minutes of a landing strip. Flights to Northern/Central Europe almost always use Gander, Newfoundland as a jumping point. The next available landing strip would be Iceland (much farther North than typical vector), if not Ireland. These planes are designed to fly with one engine. The 4 engine jets don't have the 180 minute rule, but I don't know what their rule is. I've flown often to the UK and it's like any other flight - depends on the weather at the time, but in Spring/Summer I've never had a bad flight there. The worst part is dealing with Heathrow air traffic. Personnally, I prefer London Gatwick. It's smaller and less congested. You can also fly LGW to Cincinnati, on Delta, for example which is much easier customs than Atlanta. You can't get as many flights in and out of LGW, however.
I've had completely smooth and turbulent over both. I don't think it matters. Though I think the worse place for overseas flight is the route that takes you between Japan and the Gulf of Alaska. I'm always hearing about that route being turbulent. The one time I did cross that area, it was pretty bumpy though not too bad, moderate turbulence at worse.
My flight from Vancouver to Tokyo was completely smooth both ways. Not a single bump. It was like we were parked on the tarmac for 10 hours and magically arrived at our destination. It was probably THE most boring flight I was ever on. I guess turbulence makes for a more interesting ride!