No need for any type of inoculation against exotic diseases when going to Tahiti. French Polynesia enjoys a high standard of health, with excellent medical and dental services, pharmacies, private clinics and a large hospital in Tahiti. There are NO snakes, poisonous spiders or any land critters that can hurt or sting you . It is a good idea, however to pack some bug repellent. Wear a T-shirt when snorkeling.
Beaches are all coral, with chunks which can be sharp.
Tahiti is very safe by any standard and violent crime is a rarity. Theft does happen occasionally, but you need not be concerned. Just don't be too careless. Cost and coverage may vary depending on the cost of your trip. No, you cannot be legally married in any ceremony in French Polynesia unless you have been a resident of the particular island for 30 days prior.