What about Tsunami buoys or listening devices? Thats not uncommon in that region of the world. If a plane that size were to crash down in the ocean is it plausible that a listening device or buoy could've picked it up. I'm sure it would've been a big event if it was intact when it hit the ocean.They are only detectable by sonar within a 25 mile radius (maybe even less), so it's still necessary to find the crash location first. No GPS as far I know.
Thanks for your response jaydeehess, I don't think it would be wrong to assume that in the event a plane crashes down in the ocean that nations, the US in particular would offer one of their submarines or navy vessels to canvass the area if needed, especially if they have a 25 mile range like Peter stated.