The aquarium is small but one of the best around. Yes, do it. I bought a week's pass when I was there and visited often. I recommend.
Molokini - if you have to ask that suggests you haven't been there yet.
The answer is a resounding YES. The water clarity off this island is 100 times that of anything accessible by shore.
Black Rock beach - dunno, maybe someone else can chime in on that one. Generally speaking though it's hard to go wrong at any of the snorkel spots, unless you have a bad day and the water is murky.