.

I think it depends to what extreme you snorkel. I find that if I snorkel close to shore, as long as I can touch bottom, that I do not need them. On the other hand if you are set on exploring farther out (e.g. snorkel boat trip) or if conditions are rough they are helpfull.
You should definitely bring "surf-mocs" available at KMart etc., or other water-shoes as coral and lava will injure bare feet.

Unless you only plan to snorkel for a short time in very shallow water, you definitely need them.
Last month, my husband and I - both divers - were in Cozumel. I had brought my own fins, he was renting, and as we were getting ready to fly out, he turned his in before we had one last snorkel.
He's in great shape, but lasted about 10 or 15 minutes kicking around and getting nowhere due to a gentle current before he gave up and came in.

We dive and snorkel several times a year in Maui.There is rarely any current in Kihei, nothing like Cozumel's three knot flow.
I don't wear fins while snorkeling, but I'm a strong swimmer. I can't see how the depth of the water would make any difference,you should never stand on a reef anyway.

Are you that strong of a swimmer that you can fight the ocean current without fins? If so than no, you do not need fins.

I find they're helpful if I want to dive down and get a closer look at something.

I'd say they are a must to maximize your experience. You'll cover more area and you can keep up with some fast swimming fish. Some of the best snorkeling in Hawaii is away from the beach and off the rocky points. Having fins gives me confidence that I can get close to the rocks but not get pushed into them by waves/current and I can escape any trouble easily.

I don't think there are many Fins in Hawaii for snorkeling. There aren't many of them and they usually travel in Europe.

You don't have to use fins, but... I would highly recomend them. When you snorkel/scuba, you swim with your legs only. Without the fins, you will tire quickly. I try not to snorkel without them.

I'd rent a pair just in case. They are cheap so if you don't use them, no big deal, but you may regret not having them.

I agree with JohnD, get those surf shoes in any case.