Of course it all depends on where you're going and for how long.
We've sailed around New Caledonia
for short trips (up to a month). No real problems encountered, other than one infected cut. The local boatyard guys recommended Hydrogen Peroxide for cleaning
coral/beach cuts. Dilute with hot water
and scrub out - never tried it, but it might have saved a course of antibiotics later!
We never washed our food
in bleach solutions, although it seems to be recommended by all sorts of people - we never had problems when we bought food
from local markets.
Ciguatera can be a problem in bigger fish
. Local advice is probably the best defence.
The main things we made sure to have were painkillers - simple is best. Tea tree oil
and betadine for small wounds or bites. Pawpaw ointment is good for soothing skin rashes etc. Broad spectrum antibiotics - but not much point in carrying masses of them, unless you're going to be out of the way of civilisation for months.
Basic first aid knowledge is sufficient for most problems. Dengue fever needs fluids and analgesics and rest. The more serious forms might need hospitalisation, but there is no specific treatment - prevention of mosquito bites is essential. I use tea tree oil
(not the full strength stuff) on bites to prevent them driving me crazy and becoming infected.
Anti-malarial drugs are a specialised field and need specific advice. Not all of them are effective any more, and the side effects can be horrendous. Definitely to be avoided if possible.
Oh, and have fun!!