I used gear borrowed from my work, but there is a snorkel rental place right at the beach (therefore, I have no idea about rental prices).
A group of 10 of us got up real early to head to Hanauma Bay on a Saturday and had a spectacular day. The parking lot closes once it hits capacity, so getting there early is imperative. We arrived extra early in order to get out into the water before it got crowded--and boy, does it get crowded. After watching the required educational video, we parked ourselves on the beach and got right in the water.
The water is incredibly clear and plants and animals are plentiful. We saw many varieties of tropical fish, and the coral is beautiful. The bay is protected by a natural rock/coral barrier, but if you go past the rocks, there are opportunities to go deeper and see different species of fish.
The one 'danger' to be aware of is that coral hurts! Several guys in our group went beyond the coral barrier, and on the way back, got slammed into the coral by waves and came back a little scraped up and bloody. It doesn't look bad, but in some shallow areas, you have to carefully crawl over the coral in order to get to a different area. It can be tricky and painful if you're not careful! There are many, many areas that don't require you to get near the sharp coral, though, so no worries!