Do not litter and leave behind batteries and other warmful products, as there is a high risk of being found on the ocean floor in the future.
Have a hat and a sunscreen as trips on ferry boats can be long. (For sensitive skin do not hesitate to take a t-shirt and a long sleeve sarong)
Do not go out at night without a flashlight (and yes the moon is your friend but not every night to go!)
Use anti-malarial drugs and mosquito repellent because you're exposed to mosquitoes.