Dreamliner is ETOPS-330 so it can be 5.5 hours away from an airport (I wouldn't want to be on one if an engine fails, though)

They are nowhere near gliding range to an airport. They are hundreds of minutes of flight time from landing, in fact ...

But how often do airplanes glide into the ocean because they had some failure and couldn't reach land?

Seems like an insignificant risk, and cutting the flight time by 10% might do more to reduce risk than staying over land.

Land routes also tends to have certain hazards that generally don't exist in the open sea, such as 8km+ mountains, idiots with missiles, and much busier air corridors around major cities. That might balance out the risk of an ocean crossing.

Just came back to look at this, and saw that parent had basically replaced his original comment with a new one. The original parent said that all over-water flights were within gliding range of an emergency airport at all times -- this is obviously not true, which was what I was pointing out.

Given modern aircraft design, flying over open water makes good sense.

I think there is only one case, and that was because if a hijacking. The plane broke apart on landing. It was caught on video by someone on the beach.

Of course, we don't know what happened to mh17, which might have also glided into the ocean.

MH17 was shot down over the Ukraine, you must mean MH370.