A gorgeous piece of music, "Aquarium," from Camille Saint-Saens' Carnival of the Animals:
Saint-Saens depicted underwater life as mysterious and ethereal.
Does it remind you of anything else?
Some people think Alan Menken was influenced by "Aquarium" when he composed the introduction to Beauty and the Beast. They do sound similar.
Menken (and Sebastian) took underwater life in a different direction in The Little Mermaid:
And one more underwater masterpiece. The Moldau, by Czech composer Smetena, describes:
"the course of the Vltava, starting from the two small springs, the Cold and Warm Vltava, to the unification of both streams into a single current, the course of the Vltava through woods and meadows, through landscapes where a farmer's wedding is celebrated, the round dance of the mermaids in the night's moonshine: on the nearby rocks loom proud castles, palaces and ruins aloft. The Vltava swirls into the St. John's Rapids; then it widens and flows toward Prague, past the Vyšehrad, and then majestically vanishes into the distance, ending at the Labe (or Elbe, in German)."
This video starts at the section that describes the mermaids. Definitely more on the ethereal side of water life, like Saint-Saens.