On the Big Island, aside from Two Step/Honaunau Bay which is tops, I'd recommend Old Airport from the cove at the far west end. You can snorkel either direction once into deeper water outside the small cove. Series of shoreline canyons to the the west, forming small coves to explore. This is a good spot if the winter NW swells are running, as the coast faces south there (Two Step is pretty protected in that case as well). A ways south of Kailua Town, Hookena Beach snorkeling is good heading north from the beach entry. Even a bit farther south is Kona Paradise, the cove and adjacent edges are good. I wouldn't recommend either of the latter two unless the seas are 1.5' or less (check Magicseaweed.com or another surf forecast site).
Most places on Kona, you don't want to be more than 20 or 30 yds from the lava bench shoreline, if even that. For Kona you really want to wear dive boots with felt soles and use open heel fins, unless your feet are extra tough. You can often find those boots for about $30 at Walmarts in Kailua, also sometimes KMart, but inventory can be spotty. Now and then I see them for 50% more at dive shops. The Tabi-toe low top felt boots used by fisherman can also work in a pinch.
Much of the Maui coastline, especially to the south, is pretty shallow a long ways out, but that's OK for snorkeling. Makena Beach out along the right, and any of the Wailea hotel beaches or other access points will be OK. To the north, the Crystal Gardens area south of Olawalu and Ukumehame, right after exiting the tunnel headed north, is spectacular if you can find the spot. It's a bit of a swim from entry. Way north at Honolua Bay is great as well and a little deeper.
