WHAT MAKES AN UNDERWATER THEME

[Moonlight_]

Maybe, try using some blueish instruments, like the Strings or one of the Pianos? Maybe add some reverb to the piano, and equalize the strings a little? You could use any of the Electronic instruments, you could use a Vibraphone...or you could even use the Music Box.
Your possibilities for blue instruments are very limited, though. Maybe, you'll have to use some green instruments as well, like the Electric Guitar.