I've thought about this a fair bit - when not paddling I spend my time as a rural/remote doctor with an interest in trauma and retrieval medicine. When I started out in the game I had enough kit in the ute to do a laparotomy....oxygen, intubation gear, chest tubes, splints, the whole enchilada.
Now I've distilled it down to a scalpel, some betadine and simple dressing pack, a few drips and a bag of IV fluid, plus panadol/aspirin. That's my on the day-today on the road kit unless I am formally oncall pre-hospital and driving the rapid response vehicle.
In my kayak, I'd keep it real simple - PHONE (or other device), DUCT TAPE, SUPERGLUE, ANTISEPTIC, ASPIRIN/PARACETAMOL and a MULTI-TOOL.
The way I see it, if you do anything REAL serious, you're going to need evacuation. And if the injury is real serious, you're going to need a helluva lot more kit than most would carry (oxygen for a start!)
So, you need a PHONE to get a medevac - ASAP.
DUCT TAPE allows you to splint/pad most things (and repair your busted kayak).
SUPERGLUE is great to close easy (non-tension) wounds...for anything else you're going to need sutures, which is fine if you can suture...if not -> hospital (we use special epiglu in hospital - probably costs about $200 bucks a tube, whereas can get 'non-medical' superglue from Mitre10 for $2.36
ANTISEPTIC is useful for stings etc and doesn't take up much space. ASPIRIN is best for a heart attack as well as a hangover or sprain. PARACETAMOL good for aches and pains, fever. Black n Gold brands are as good as any other and cheaper.
MULTI-TOOL is great for cracking beers, fixing rudders (so use a skeg), shucking oysters, fighting off raft guides and of course doing an emergency cricothyroidotomy
My point is, I can cram my kayak with all gear I need to patch someone up in the Emergency Dept or when I am doing prehospital work. But, in that case, I'd rather opt for doing the basics well and acting quick to organise a retrieval, rather than piss around on the beach. And keep my kayak empty for food/alcohol/camping gear
And if the victim is not that sick, superglue, duct tape, multitool plus phone/antiseptic will deal with both human and equipment failures and allow the trip to continue!
Someone usually asks about D&V and rehydration. Ginger and kwells don't really cut it. We have some excellent wafers of ondanseton (a powerful anti-emetic used in chemo) which work well. And rather than piss around with IVs and bags of crystalloid, better to stop, try small sips or, if all else fails, rectal rehydration (don't ask!)