Plastic static wrap is a good thing for closing up large gashes (think prop hit or shark bite.) Not exactly sterile, but better than bleeding out. I get the 4 or 5 inch wide roll from a hardware store, and repackage what I carry on board onto a dowel (pencil would work fine.)
I don't typically carry splints that would set a leg, but have enough stuff to improvise if needed (paddles, spear shafts/guns, boat hook, etc.) Lots of tape in the kit helps here.
As a day trip diver, I stock a lot of stuff for people who don't feel well: seasickness meds, pain relievers, salves, antacids, antihistamines, etc; and a lot of stuff for minor injuries: bandaids, tape, finger splints.
Major injury requires return to the dock or air evacuation (or at least some sort of higher level care), so just enough to hold out until help gets there is fine.
One thing I don't have is an Epi-pen (injectable epinephrine.) Should have it in case of a severe reaction to, say, jellyfish toxin or similar; haven't needed it so far, but would be good to have. Anaphylaxis can kill pretty quickly.