I'd say start with a good kit and add to it.
Make sure to add stuff for Jelly Fish stings, man of war, sun burn, etc.
Add band-aids that will stay on wet body parts, like the cloth ones. Add surgical tape and duct-tape with sisscors. Liquid band-aids are good for blisters on liveaboard trips as is "mole skin" Other creams for sea rash and that dreaded "wet suit rash". alcohol, etc. meat tenderizer and baking powder are good for stings, as is tobacco.
A good pair of maginfying tweezers. waterproof ziploc bags (gallon and quart) size. Freezer is best since they are stronger. surgical tubing for tourniquit(sp?). Also is a good backup for replacing a Hawaiian sling surgical tube that breaks.
Best of all, get a good waterproof case like Pelican or other to put it all in. Not only will this keep water out, but sea/salt mosture also. Chunk in some of those silica gels things that come in electronic products that say "do not eat" to soak up moisture also.
A pre-paid cell phone (and 12v charger/adapter) are always a good idea and not tha expensive. A handheld VHF (and 12v charger/adapter) is also nice, but you're looking at around $100-$150 easy there.
Don't forget sea-sickness medicine either.
Pen and paper, in ziplock bags. Important if you have to evacuate someone you need to write on it what time you think they went down, when they came up, time ,etc. IF they get put in an evac helicopter or ambulance this info is important to the emergency room doc.
One more thing I thought of. A good medium sized flashlight with extra batteries. You might be out for a day dive that turns into a night situation.
hope all that helps.