This is similar to the old "cave sounds are too frequent" bug during the betas.

This isn't major or high priority, but more an annoyance. When playing the game, it's common to jump into water and become submerged for a short time, before coming back up to the surface. However, the issue here lies in the fact that the trigger for the underwater music is set instantly, causing currently playing audio to fade out, and immediately be replaced by the underwater music. This can be annoying when simply traveling, as music may be playing that you enjoy if you're using a resource pack, or if you simply enjoy the default music, and an unfitting theme will begin playing as if you're exploring underwater. There should be a delay of a few seconds before the under water trigger is set, so that accidentally submerging doesn't cause a complete shift in ambiance.