Sharks cove was the best spot we found. There are a lot of fish at Hanauma, but it was packed with people, and the visibility was pretty poor.
Sharks cove has a ton of fish, all different types and extremely clear water. Off to the left side the water level is very low, but there are still a lot of fish. It can be as shallow as 1 ft or less, but up to 10 feet. Out to the middle or off to the right I would guess that it is about 30 feet, but very clear. There were hundreds of tangs here, and we saw a couple puffers, and an eel as well as many other fish. Oh yeah, the clairity is better earlier in the day also. It did get a little stirred up when more people showed up.
Here's some of the tangs that we swam through
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This shows some of the variety of fish
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Here's a couple out of the water. It's beautiful to look at even before you get wet. Hopefully this will give you good shot of what to look for when you get there.
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If you continue up the north shore just little further you'll get to Turtle beach which we saw five turtles up on the sand both days we were there. A little bit further is Sunset beach which has some really nice waves, and beautiful beach. If you head to the other end of the north shore (left instead of right when you get towards the top) there is a hidden beach which is out past the airport. It sounded pretty promising, but we were unable to access it because the dirt road at the end of the highway was flooded from recent downpoars.
Hope you have a great trip!