When you are out on your boat, there is no well-stocked pharmacy close at hand and you are beyond the reach of quick-response medical assistance. You will be helpless to deal with onboard illness or injury unless you have medical supplies aboard and the knowledge of how to use them. Most medical "emergencies" are minor--cuts, burns, sprains, stings, and punctures, to name a few--and if you have the appropriate bandage, ointment, cold-pack, medication, or tweezers aboard, you can usually take care of these events. Otherwise your boating day is over.
Think of a first aid kit as a tool kit for keeping the crew in working order. You can assemble your own kit or buy one that is prepackaged in a waterproof box or bag. What should it contain? That depends on how you use your boat. The farther from shore you venture, the more self-sufficient you will need to be.
Don't overlook the fact that you could be the one ill or injured. Provide a visual clue of where you have medical supplies stowed by marking the outside of that locker or cabinet with a prominent Red Cross symbol. Keep all your first aid supplies together, not distributed around the boat. Having them in a box or bag lets you quickly take the whole kit to the victim.
Absent medical training or a doctor among the crew, you will have to rely on a first aid manual to show you how to handle any unfamiliar emergency. Be sure your medical kit includes a good first aid manual. The first aid manual belongs with the medical supplies, not with the mystery novels and cruising guides.