The Waikiki Aquarium is managed by the University of Hawaii and focuses specifically on Hawaiian and Pacific marine environments. It is modest in scale compared to major mainland aquariums but stronger in relevant content — the coral reef exhibits recreate the actual ecosystem of the lagoon just outside, and the Hawaiian monk seal pool allows close viewing of one of the world's most endangered marine mammals (around 1,400 remain in the wild).
The cephalopod display — nautiluses, octopuses, and cuttlefish — is excellent, and the outdoor living reef section is a good primer before snorkelling. Admission is modest and the aquarium is small enough to visit comfortably in 90 minutes. It sits at the Diamond Head end of Waikiki on Kalakaua Avenue, directly across from Kapiolani Park.