You would probably want to have some narcotic-based pain medication on-board in case of serious injuries. Tylenol-3, which contains codiene, Percocet, Dilaudid or morphine sulphate, are possible choices. I'd stay away from Oxycodone or oxycontin, as that drug is flagged more by DEA due to it being "hill-billy heroin".
It would also be good to get another broad-spectrum antibiotic, since many diseases are now fairly resistant to pennicillin.
If you're going to get any of the medications in tablet form, make sure that you can get them in smaller containers. That way, if you do have to use the tablets, you can keep some of them sealed. Tablets tend to degrade in high-heat/high-humidity conditions, fairly rapidly.
Also, Silvadene creme is another thing you'd probably want to have on-board. It is used for burn treatment.
Prescriptions for antihistamines don't really make much sense... Benadryl, Claritin, and Sudafed are available over the counter...and all three are quite effective. Benadryl also works partially to counter motion sickness, but can make many people drowsy.
One other prescription medication that may be of interest, which I believe is available transdermally, is Zofran... it is a very potent anti-naseau drug... can be of great use in combating the effects of seasickness.
You may also want to get, depending on where you're planning on going, some form of anti-malarial medication. Be aware that in many areas, the older standard treatments for malaria are no longer effective.
Another medication you may want to carry is an anti-parasitical medication... Tapeworms and other intestinal parasites are fairly common in third world countries... you may want to discuss this with your doctor.
A few other things I'd pack in the Medical Kit, and I call it a medical kit rather than a first aid kit, since you will probably be further than 10 hours from medical assistance...so it really is a medical kit...
1) Eye wash of some sort
2) Boric acid powder, which is both a good insecticide and topical antiseptic.
3) splinting equipment—SAM splints are excellent, and fairly easy to use
4) Antifungal lotion or creme—Tinactin, Lotromin or something like that.
5) Electrolyte solution powder for rehydration
6) Multi-vitamins for daily use. Prevents scurvy, etc.. and easier than carrying lemons and lime juice.
One other piece of advice.... I would take the bottles of medication and then individually vacuum seal each bottle, using a food vacuum sealing machine. This will protect the medications for oxygen degradation as well as humidity degradation... Then you can store the medications in the bilge, where it is coolest... without worrying about the effects of any bilge water on them. Don't do this for any transdermal patches, as the vacuum sealing process may damage the transdermal gelatin matrix. It should be fine for any tablets or caplets though.
One last thing I'd carry is some Cyanocrylate-based "crazy glue" type adhesive. It can work to seal larger lacerations fairly well. Medically, it is sold under the name Dermabond, but I've found that the regular, hardware store supplied stuff works just as well. It is more effective and less prone to problems than either sutures or surgical staples, at least under the conditions you'll be using them.