Just as a matter of interest, has anyone ever compared the long over-water routes (eg Aus to South America) to the shipping lanes and how busy they are?
If in the unlikely scenario of a flight coming down in the wide expanse of the Pacific, one's chances of survival would be moderately enhanced if there was a steady stream of freighters sailing past to effect a speedy pickup.
However I'm guessing that if the aircraft's route requires a lengthy ETOPS rating then that naturally means that there's never a whole lot of anything (let alone a usable airstrip) down on the surface under some parts of the route.
You are correct.
The map you are looking for is here. As you correctly assume, the really, really long ETOPS routes run across stretches of ocean that are largely devoid of shipping.
In any case, having traversed the Southern Ocean in a ship a couple of times, let me tell you, it doesn't matter. Forget about any successful ditching in these waters, ain't gonna happen. This is not the Med....