Mains water is safe to drink but some visitors do get mild abdominal upsets. There is high calcium content in the water, which can result in a few white specks in tea or coffee. Bottled water is inexpensive and readily available. It is important to keep well-hydrated in the tropics. Most milk is UHT/long life milk. Local meat, poultry, seafood, fruit and vegetables are considered safe to eat. The local beef (organic) is excellent.
While there have been no recorded cases of malaria on the main island of Efate for many years, for travel to some of the outer islands, anti-malaria medication is recommended. Consult your doctor/physician. Your doctor may also recommend a vaccination against hepatitis. A tetanus injection can be required for some cuts/abrasions but this can be had after the event. Coral cuts are common and they can become infected. Treat with hydrogen peroxide, antiseptic and keep covered.