Underwater sound travels four times faster than in air, which is why a humpback's song carries miles — the ocean is a better conductor than any concert hall. These 304 fish and aquatic recordings work that medium: whale song with the long ascending phrases, dolphin click trains used for echolocation, orca calls from a Pacific pod, bullfrog bass from a Louisiana bayou, and a section of frog croaks from a pond at twilight.
Nature documentaries pull from the whale song because it's the single most recognisable underwater sound in popular culture and audiences read it instantly. Children's animation uses the dolphin clicks for cute aquatic characters. Horror and sci-fi creators take the deep bullfrog bass and pitch it lower for off-screen creature menace. Meditation channels loop the frog ambience for night-time forest soundscapes. The whole aquatic set is free MP3 download, no signup, no attribution.