This answer depends on whether the plane "crashed" into another object - or exploded - and fell out of the sky (unlikely), or if there was a mechanical problem that would require a "ditching", or a landing into the water (more likely, but still improbable). Flight Attendants are trained to deal with emergencies like this during initial training, and the procedures are reviewed on a regular basis as well.
The first thing that happens is that the Flight Attendants will review safety procedures with you (seatbelts on, brace positions, and operation of life vests if onboard, etc.), and then go through the cabin, checking for passenger compliance and briefing passengers in the exit rows on what to do in the event of an evacuation...
After that is done, they will stow all of the loose objects that are in the cabin, and lock down the galleys and lavatories. After they finish performing ALL of their safety duties, they will communicate with the Flight Deck and let them know that the cabin is secure.
In a ditching, the plane sort of "glides" into the water. The Flight Attendants will be trying to ensure everyone's safety as this goes on. The aircraft is actually able to float, so after the aircraft has come to a stop and the Flight Attendants are given the signal, they start commanding an evacuation.
The rear doors are (usually) not used in an evacuation, as they are below the water line (normally). However, all of the other doors are normally free from obstructions, and are able to be used. The slides will deploy, and may be used as additional floatation devices. Passengers will be evacuated into them as quickly and safely as possible.
One special note: do NOT inflate your life vest until AFTER you are outside the aircraft. This is very important for your safety.
Once everyone has been evacuated, the normal procedure is to attempt to tie the life rafts together. This makes the rafts a bigger visual target, and allows supplies to be shared between everyone.
The aircraft that are equipped with life vests (aircraft capable of travelling more than 50 nautical miles over water) and life rafts (planes that are designed to travel more than 100 nautical miles over water - read about life rafts here: http://www.winslowliferaft.com/fars.asp) have enough safety equipment (besides that seat cushion) for every person on the airplane. The famous seat cushion that the flight attendant drones on about at the beginning of every flight will stay afloat for 8 hours.
Most likely you would be rescued very quickly. There are several other safety devices that can be used to signal rescuers and guide them to your location.
I hope that this helps answer your question.