All of the music in this game was composed and performed once again by Taito's in-house (and kick-ass!) band, Zuntata. One of Zuntata's earliest works includes the music for the 1986 classic Bubble Bobble.
Evidently, Taito seems to have a thing about the aquatic theme you see in Darius; most of the enemy ships and all of the bosses are modelled after some form or another of marine life. This has been a staple in the Darius series since the first game, much like Konami's Gradius series and Easter Island heads.