Oh, it is so cheap to rent the snorkel gear there. I think my husband and I spent $25-$35 total for both of us for over a week. I have my own snorkel gear and didn't even bother to bring it because of suitcase space. The equipment you rent is great and sanitary, and the rental shops are EVERYWHERE. But if you prefer your own gear, then you will save a little money.
A tip: If you go on a tour to Molokini, take the afternoon tour. (I did the Tradewinds II and it was great.) It's about a third of the morning tour cost and a fraction of the people. On a boat that held 200, there were only 35 on our boat, giving us plenty of room in the water and on the boat. (They also offered the snuba that you were asking about.) They try to tell you that the morning tour offers "more fish" but that's not true. Also, the water clarity can be better in the afternoon.
Another tip: Go to Black Rock! (Maui) I cannot recommend it enough. It's just a free snorkeling spot on a small beach, but by far the best snorkeling I did in Maui. We went back everyday.
Hanauma Bay is a great spot to start, if you are going to Oahu first. You see a little movie first that identifies the fish for you, and is a great place for beginners. Shallow water, nice beach for a picnic, and world-class snorkeling.
Lastly, if you are into a thrill, try snorkeling with galapagos sharks (in a cage) on the North Shore. It was an experience I'll never forget.
Have a great vacation. I am green with envy at all the great snorkeling you are going to do!