The aspect of snorkeling that gives people the most pause is swimming with their head in the water and breathing through the tube. If possible, it helps to practice in a calm body of water, such as a swimming pool, to get a feel for how it works. Pick up a snorkel set and get familiar with it.
Another thing that will help you be a better snorkeler is to become a better swimmer. Practice swimming with your head in the water, then stop and tread water for a while, and then start swimming again. Use a flotation device for extra support until you get comfortable without one. Increasing your stamina and endurance in the water will help you enjoy snorkeling in Hawaiʻi. Start out in a calm swimming pool.