I think also, once you get the hang of breathing through the snorkel you will be just fine, especially with a flotation devise on, which I recommend. I always wear one, slightly inflated and I can swim around for a long time with no trouble.
I'm sure there will be plenty of opportunities in Hawaii to snorkel, enjoy!
Snorkelling is easy...as long as you can breathe through your mouth. I started diving in the 50s and have been totally at home in the water ever since. Unfortunately my wife gets claustrophobia in a mask. She just can't get used to breathing solely through her mouth.
So if you're at all claustrophobic, you might want to check with a local dive shop that has a pool available. Most offer free intro sessions where you can try out snorkelling and diving. They also have classes from the most basic.
As to free diving...that's diving with mask, snorkel, and fins only...boith men and women have been down over 200 feet. You simply have to learn how to adjust the pressure in your ears.
There are lots of great places to snorkel in Maui. My personal favorites are on the SW coast...Wailea Beach, Maluaka Beach, down to La Perouse Bay...great lava tubes, lots of turtles, and more fish than you can count. Used to be a balck tip shark in the lava tubes affectionately known as Julia but I'm sure she's long gone by now.