> It's obviously at the bottom of the ocean.
That's the default expectation in this kind of situation, but in this case it's actually slightly hard to see how it could have happened. The evidence suggests that when it first "disappeared", and likely for hours afterwards, the plane was under controlled flight by someone with at least some idea of what they were doing. Whoever was flying the plane could easily deliberately crash it at any time while flying over the ocean, of course, but if their intention was just to crash the plane somewhere non-specific, why not just do it right away or soon afterwards? So they very probably had other intentions.
And an airplane in a controlled cruise over the ocean is unlikely to just happen to crash into it by accident. It will obviously run out of fuel at some point, but the people flying the plane almost certainly planned to do something interesting with it before that happened: the likely candidates are a) land it or b) crash it into something notable. They probably had the ability to read the fuel gauge and/or an a priori idea of what the available range was, too. Now there are all sorts of ways their plan could have gone wrong, or could have succeeded by crashing the plane, but they nearly all involve the plane landing and/or crashing on land, or relatively near a coastline on the approach to land instead of the deep ocean. AFAICS it's easier to list ways in which the plane could be lost in the deep ocean:
1) A Flight 93-style battle for the cockpit erupts while 370 is over the ocean
2) The person flying the plane is a terrorist/hijacker/pirate who lacks real airliner flying expertise but has been equipped with a cheat sheet telling him in detail exactly what to do. Everything goes fairly well for a long time until something unexpected happens and/or he messes up one of his scheduled actions. At this point he likely panics and maybe does something rash
3) The people in control attempt a controlled ditch somewhere in the ocean, probably in order to rendezvous with one or more speedboats and transfer themselves and some or all passengers. This could very easily go wrong, but succeed or fail the plane will probably end up sinking either way
4) The people in control attempt to strike out across the ocean for some distant target which they guess is near the limit of their range. But they miscalculate, and by the time they realise it they're trapped over the ocean with no path back near land in time
Scenarios 3 and 4 are fairly unlikely if you believe the radar interpretations suggesting that the plane went west across Malaysia and then north, though.