I'll chip in...I'm a nurse who works in the Emergency Department and ICU, formerly did fire and ambulance for 25 years, taught EMTs, wilderness courses, industrial courses, combat first aid (SABC) for the Air Force, and others.
A lot of what you might need is dependent upon where, when, and with whom you'll be boating. Inland boating, coastal cruising, or blue water voyaging? Solo, with family and/or friends, with anyone with special medical needs or certain medical histories?
For example, an AED is, indeed, a potentially life saving device, but its application is extremely limited in the backcountry...or far at sea. I was once the naturalist guide on a trawler yacht with 16 tourists aboard, 600 miles at sea, when a passenger had a heart attack. If he had suffered cardiac arrest, no amount of CPR or defibrillation would have saved him as we were days away from rescue, with no helicopters available (in the Pacific off Ecuador). On the other hand, were we in coastal waters of the US, with possibility of timely helo medevac, it would make more sense to have these capabilities.
Will you have a passenger with brittle diabetes aboard? Glucose would rise to the top of the list of medications to have aboard. Do you have a crew member with a seizure disorder? Suddenly, you wish you had a Diastat injector...rectal Valium. Family member with a history of coronary disease? Chewable aspirin, sublingual nitroglycerin, MAYBE an AED. Severe allergies? Definitely two EpiPens...or, for far less money, a couple of vials of epinephrine and a couple of syringes, if you want to get trained to do it manually. I would add Benadryl and Pepcid/Zantac as lifesaving meds for those with severe allergies, to augment the epi. Cruising in areas where encounters with jellyfish might be likely? Keep vinegar, antihistamine, and a source of hot water aboard. By the way, the only possible..and it's a stretch...circumstance in which I'd consider having oxygen aboard might be if a lot of SCUBA diving were being done. Speaking of which, if you have a dive compressor on board, do you know how to treat barotrauma?
So a lot of what you need may be anticipated, depending upon where you'll be and with whom.
A mistake I've seen over and over is folks building a kit...trying to be comprehensive, anticipating any possible disaster...but not building their knowledge or skills. Don't bring equipment for emergency needle thoracentesis if you have no way to use it! What we always emphasized in our wilderness courses was improvisation...how to make and apply bandages and splints rather than how to use a lot of specialized equipment. For example, if your head-injured patient can't maintain his airway while awaiting rescue, you could perform endotracheal intubation (not), insert a nasopharyngeal or oropharyngeal airway (only if you have them and know how to use them), or you could safety pin his tongue to his lower lip and position him in the recovery position. Caring for a blistering burn? Please learn how. Hint: no butter!
It comes down to training, training, training. With thoughtful training, you're better off with some homemade triangular bandages, sanitary napkins and Gorilla tape than the untrained person is with the EMT's kit off the ambulance. Resources for first aid training for boaters that come quickly to mind include your local Power Squadron, Propeller Club, Coast Guard Auxiliary, DANBoater, the Cruising Club of America, Sea Scouts, and local community or technical college. If you add wilderness first aid training, which is the same stuff in a different venue, the options grow. Finally, one course can't cover everything, and you can't remember everything forever. It's important to have a good reference aboard, a text book that addresses your needs. There are little pamphlets and flip books, paperback books, and large hardbound texts. Select one based upon all the above considerations. For example, the lay public seems to have no idea how to effectively care for a nosebleed. Here's how: pinch the nostrils tightly and tilt the head slightly forward. Wait a long time. If this doesn't work, blow out the clots and spray heavily with Afrin. Pinch and wait. If that doesn't work, repeat with more Afrin, pinching and waiting. If that fails, blow out the clots, saturate gauze in Afrin and pack the nostril. Pinch and wait. Keep pinching and waiting. Of course, you could always Google this stuff...oh, wait...no cell service? Bummer. There are lots of books that address everything from simple first aid to complicated diagnosis and treatment.
I like to assemble supplies and equipment for several basic purposes.
1. Routine care: treatment for boo boos such as cuts and scrapes, sprains and strains, sunburn and seasickness, fevers, simple allergies, rashes and itching, headaches and belly aches including heartburn, diarrhea and constipation, etc. Throw in stuff as needed for specific crewmembers, such as migraine meds, rescue inhalers, glucose gel/tablets, cream/suppositories for hemorrhoid flare ups, etc.
2. Outpatient care: treatment for things that ordinarily might require a walk-in clinic, such as laceration repair, severe sprains & simple fractures, draining an abscess, repairing a broken tooth or lost filling, foreign objects in the eye, deep puncture wounds/impaled objects, etc.
3. Oh **** care: this depends upon a lot of those factors mentioned above. I don't need a heavy trauma kit and morphine if I'm within 911 coverage, but would include these on a blue water passage.
Over the years I've had as much opportunity to help others with my knowledge and supplies as I have myself and my family. For this reason I carry naloxone (Narcan) on board, as well as epinephrine and a few syringes. These are highly personal choices not suited for everyone. More likely, you can be the hero with some bandages, antibiotic ointment, cortisone cream or Tylenol.
A good friend of mine who is a CT tech at the hospital, an EMT and the medical officer for our fire department, is an avid boater. Unfortunately, he's also somewhat accident prone. He once received bad burns while using a heat gun to speed the evaporation of cleaning solvents from his diesel tank. He couldn't really help care for his injury, but had the stuff aboard to make it work and could guide others in using it. Another time, using a dinghy lift, he amputated a finger at the middle knuckle. This time his wife, a nurse, was aboard and could quickly control the bleeding, apply a secure dressing, give him some appropriate pain meds, and care for him when he grew faint. He got great care from her; his chief worry during that adventure wasn't losing his finger but the way that she slammed the transmissions in and out of gear without reducing throttles!
Lastly, have the appropriate communications gear and know how to use it. If you're on the ICW, you can't necessarily count on your cell phone. If you're taking the inside passage in British Columbia or Alaska, you can't necessarily count on your cell or your VHF. Is your MMSI current and correct, i.e., not registered with a previous owner? Is your PLB/EPIRB properly registered, service plan active, and/or battery charged?
The main philosophy I use when anticipating such needs is something said over 100 years ago by Louis Pasteur: "chance favors only the prepared mind."