1) Coral cuts - lemon juice (yes it hurts like hell) while they're still fresh then scrub the wound hard with soap and water. Keep the cuts clean and covered, stay out of the water and change the bandage after showering every day.
2) Swimmers' ear prevention - after swimming in salt water always rinse your ears out with fresh water then dry them out with a towel. This gets the salt off and it's the salt that keeps the ears moist and causes infection.
3) Stone fish stings and centipede bites - apply the hottest thing you can handle immediately to neutralize the poison. I know a guy who didn't have to go to the hospital after stepping on a stonefish because he put his lit cigarette out on the puncture. Better a scar than long-term pain and possible death.
4) Foot fungus - soak foot in hot water with a cap-full of vinegar in it for about an hour.
5) Staph infections on the feet - soak foot in the hottest water you can stand with a cap-full of bleach in it till the water turns cold. Then clean off all the goop, bandage and keep clean and dry. This works great on any infections that can be soaked.
6) To get rid of head lice - saturate your head with olive oil and leave it on about 5 hours. This works better than any commercial lice shampoo even though it's messy and a complete hassle. A few drops of lavender, citronella, thyme and/or eucalyptus essential oils in the olive oil help too.
7) Cockroach infestations - put out boric acid anywhere domestic animals can't reach it then put out 'beer traps' - about an inch of beer at the bottom of a slick bowl. The roaches climb in, get drunk, can't get out and drown - swear to God.
8) Sunburn - aloe is great but Tahitian Tamanu oil works wonders if you can find some.
9) To keep away mosquitoes and noseums at a picnic - build a smoky fire. This is pretty effective but you have to keep at least semi-inside the smoky part for it to work.
10) Getting laughed at by locals for being afflicted with any of the above problems - drink the rest of the beer you didn't give to the roaches and open cold fresh ones for the rest of the crew. Laugh and enjoy.
Note: I'm not a doctor but these tricks have worked for me. If you squirt lemon in your eye instead of your coral cut, accidentally pour beer in your ear instead of water or do anything else stupid (or not) because of this list, I am not responsible.
Also the above photo is of my son Tevai, lover of centipedes.