In the tropical climate of Hawaii, it's possible to swim and snorkel all year round. However, summer is considered to be the most favourable time for these activities. The water temperature in mid-summer reaches 27 °C, so you can stay in the water for an hour or more and feel comfortable. In winter, the water is usually no warmer than 24 °C, which is not that comfortable if you want to stay in without a wetsuit. Also, summer gets significantly less rain and the wind, which lets the water remain calm and clear. So if your primary goal in Hawaii is snorkeling, choose the summer months for your visit. Hanauma Bay is reportedly one of the best snorkeling locations in Hawaii. The water is very calm and clear due to the shape of the bay, and coral reefs are abundant with fish so don't forget to bring your underwater camera.