There are many different types of emergencies at sea, but I'll just discuss the most common one and some of the safety devices involved.
First of all you should have some type of crew card on the boat that is easily accessible and lists each of the crew members duties during an emergency.
In case of a man overboard the person who first spots the MOB should point to the victim and continue to watch him until a crew member can take over. Life rings or other bright colored items should be thrown where the victim is seen. The crew member should try to be in a position on the ship where the captain can see him/her. The captain will then maneuver the ship back towards the MOB. Two other crew members should be preparing the block and tackle on bigger ships and on smaller ships they should be near the back of the boat. When the bigger ship is along side the MOB they will toss a ring that is hooked to the block and tackle and then work together to haul the MOB out of the water. On the smaller ships they will use the diving board to haul the MOB in.
There are many different types of life jackets, but be aware that not all of them are Coast Guard approved. The first type is the Type 1 PFD, or personal flotation device. The type 1 is meant for open, rough or remote water. It will float you the best and turns most unconscious wearers face-up in the water. The only disadvantage is that it is bulky. The second type is a Type 2 PFD. These are good for calm, inland water. It will turn some unconscious wearers face-up in the water and is more comfortable than a Type 1. These are the two most common PFD's, there are also inflatable vests, or Type 5 Hybrid, and Type 4 throwing devices.
Wear a Life Jacket whenever you're on a boat. Almost nine out of ten drowning accidents happen to people without life jackets.