In recent snapshots, I've noticed that submerging into water causes the background music to suddenly stop, and not begin again until the next track plays. It breaks the general immersion of the game somewhat, like a more frequent version of MC-36501.  It does, however. make me think that some kind of muffled effect could be applied to music when you go underwater (at some point later in development). This would be a much nicer effect as it helps with the general idea of the undersea exploration almost being a different world to the land-based gameplay