underwater aquarium
Refers to an aquarium facility where visitors can view marine life in an underwater setting, often with transparent tunnels or submerged viewing areas.
At the new underwater aquarium, you can see fish while walking through a tunnel.
水族館 is a general aquarium, while 海中水族館 specifically emphasizes an underwater or submerged viewing experience.
Compound of 海中 (かいちゅう, 'in the sea') and 水族館 (すいぞくかん, 'aquarium').