Said plane sends off Mayday message prior to ditch landing
Upon immersion into water, water activated Epirb (polar orbiting statellite transmittor) is set off
Who ever is left alive, open doors and emergency actived slides hopefully open
At bottom of slide, people get into the life rafts attached
Once all alive passengers are out, the Coast Guard or Navy (depending upon what waters you are in) have gotten the signal off the satellite and are already coordinating with Mayday signal to find location.
Upon finding the 3 mile radius of location, all ships in the area recieve the SOS signal from the Coast Guard/Navy
If within helicopter range/spotter plane range, aircraft will be scrambled to make eye contact with surviors.

Total time to be rescued: Between 3-24 hours.

Usually, if flying US-Europe, there is enough freight traffic that a ship is within 2-5 hours away from you. That's going to be the first to usually arrive.