New Caledonia is fairly safe, but it is wise to take the following precautions:
When snorkeling, avoid contact with sea urchins, which are often poisonous, and coral structures, which can cause scrapes that swell badly and take a long time to heal.
A seasnake known locally as the Tricot Rayé has a potentially lethal venom, but is not aggressive when left alone, and only attacks when threatened.
There are sharks, some of them quite large, though Great White Sharks are rare. Avoid shark attacks by: Not carrying fish that you have caught and may be bleeding while in the water Facing the shark, so that to the shark you appear large, vertical and difficult to bite
There are no crocodiles native to New Caledonia. Rogue individuals have been observed on the island no more than twice within the past 200 years, probably swept out from the Solomon Islands.
Iodine or a similar disinfectant is invaluable to fight off small infections, which quite commonly occur in most sores and scratches.
Some mosquitoes carry the dengue fever virus. There is no vaccination for this. Consult a doctor for more information, and see the Wikitravel article.
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The official language is French, and it is difficult to find English speakers outside of Noumea. In Noumea, French, English, and Japanese are widely spoken at hotels, restaurants, and shops. To enjoy a place like this, you should really endeavor to learn some French.