You've read what to look for and you've chosen the places you want to see. Try snorkelling at low tide, preferably in the morning. At low tide, the reduced wave motion causes less disturbance in the water, which ensures better visibility. You can also avail the benefits of Best Kona snorkeling.
Before getting into the water, plan out the paths you want to snorkel and remember that for every stretch you take outdoors, you'll have to swim back a little more tired than you did at the start. Plan your trip for the weakest swimmer in the group so there is some level of comfort at all times.
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Be sure to look at the shore while snorkelling and assess your position. The relative ease of snorkelling and the ability to actually glide for hours without lifting your head out of the water can put you in uncomfortable situations or downright dangerous, like losing your friend out of sight!
Once you get used to the equipment and have developed good comfort in the water, you may even want to try spearfishing you see at your local fish market! Either way, snorkelling is more than just a fun day out, it's a series of excursions that can make your time in Hawaii stand out at a fraction of the cost of some of the "touristy" activities.
After a week of snorkelling, you and your family will probably feel like seasoned beachgoers on the island when you realize how little you've spent on your neighbourhood adventure in Hawaii!