Is a scenario like the one in LOST possible? Is there a possibility of finding a significant number of survivors from a plane crash in the sea? Is there evidence of anything like this having ever happened in our world's history?
This question actually has two parts, one obvious one is if it's possible have survivors after plane is ditched into the sea. Wikipedia provides dozens of referenced examples of such a cases, two prominent ones are already mentioned in Malvolio's answer.
Other part is "LOST" scenario is possible, which I understand as in passengers surviving but disappearing without a trace. This is highly improbable, due ICAO SARPs requirements for ELT (Emergency Locator Transmitters) on aircraft itself and on rescue equipment such as rafts. These ELTs operate as part of Cospas-Sarsat system. The only feasible scenario where none of the ELTs would be functioning, is aircraft wreck sinking into deep sea, dragging all of ELT equipped survival equipment to the bottom with it. In which scenario chances for survivors are practically nil.