I like coral walls that start near the shore & near the water surface. The best one I have snorkelled is in front of the Coral Princess resort, which is approximately two miles north of town in Cozumel, Mexico. From the shore, you jump into the water and right there, directly in front of your eyes, is the coral wall. The current is mild and runs approximately parallel to the shore so you can see the whole wall without having to move a muscle. Just let the current take you. I did not Scuba in Cozumel.
The second best wall that I've seen is in Dahab, Egypt about a quarter mile south of town. There's a backbacker camp on the shore where we stayed in 1999 for four or five dollars a night, breakfast included. The only problem is that at low tide you have to walk on top of the coral reef for about fifty yards before you get to the drop. But once you get to the drop, the coral is brilliantly colored and teeming with life. Dahab is a Bedouine village full of young foreign backpackers and divers. The locals ride camels as well as cars up and down main street. The scuba scene in Dahab is world famous and well-reported elsewhere so I won't waste any time repeating what's already been said.
Lizard Island, Austrailia (accessble by air from Cairns) does not have a coral wall. However, there is a cove south of Cooks Look that is filled with giant clams. You can either spend a fortune and stay at the ultra swank resort over the hill and further south, or you can spend next to nothing and stay at the primitive campsite that is directly on the cove. The best Scuba dive off Lizard Island is the one with the Giant Cod. You have to sign up for it at the ultra-swank resort, however.
The snorkelling cove by Diamond Head on Maui is nowhere near as good as at the sites mentioned above. I haven't done any Scuba in Hawaii.
As far as fresh water snorkelling goes, you can't really beat snorkelling with the manatees in Crystal River, Florida. If a turtle impresses you, then you'll be blown away when you see a sea cow.