If you have an emergency, make sure you have a plan on your boat or even on shore. At work we have dive protocol that was set up by our board and our Dive Safety Officer (DSO). This may not be what you want to do but it will give you an idea. We dive on scuba a lot and once had a scare where one of our divers was doing a deep collection and thought they got bent. I've also been hit by a boat prop and have been to the ER so many times for work that the guys there knew me by name.
On the boat we make sure everyone knows how to use the GPS and VHF radio. If there is an emergency we immediately hail coast guard on Channel 16 and let them know our location. We were advised by our DSO that if we are diving offshore on scuba that if there is any reason where we know we need to use the chamber that we notify coast guard that and immediately head to Catalina. Otherwise we wait for the emergency personnel to make the decision.
For any other dive related emergencies we make sure we notify coast guard and try to get in touch with the county lifeguards if we are diving work hours. We have been trained on CPR and oxygen administration and keep an oxygen tank on our boat.
For most of us guys that freedive make sure you have a basic plan for emergencies:
Make sure everyone can run the boat, safely start it, put it in gear, etc.
Make sure everyone can read the GPS location numbers or find them and use channel 16 and how to call in a mayday distress. You will need your location, nature of the injury/disaster, and age/description of the victim.
Make sure your VHF is set for US frequencies, I had not realized that my older unit came preprogramed with a different frequency until my buddy tried to hail me when he was right next to me.
Make sure you know where your first aid kit is, and it's a good idea to take a first aid class if you have not done so before.
If you are shorediving, you should be doing it with a buddy. You should have a general plan of when you are getting out and way to contact each other. One of the stupidest things that I do is dive alone, or dive with a buddy and leave at different times. You should never do this, but if you do, you should have a contact person who will know to look for you if you don't return at a certain time and where you are diving from.
There are so many scenerios that I can't cover them all, you can cut yourself on a dive knife, get beat by the waves on the reef, hit by a shark, shallow water blackout etc. Just try to cover your bases.