June is the hottest month of the whole year. The ultraviolet rays are quite strong, especially at noon, so you are not advised to go outside at such a time. It will be cool in the early morning and evening because of the sea breeze.
Wear T-shirts or vests with shorts, and you'll need another long-sleeve shirt since it is cool in air-conditioned rooms. Also protect yourself from the sun and use sunscreen, and take an umbrella during rainy days.
Because of the hot weather, it's not the busy season for traveling in Sanya, but you will find it more affordable and can enjoy a less bustling atmosphere. See our Sanya Tours.