My little har-Monica shark. Awww!

Oooo shit how did we forget her @13579a

The real questions

Well do you play harmonica with your mouth or nose?

Neither since you can't play a harmonica underwater using water. The water won't vibrate the reeds fast enough to produce the sound. You'd have to be pushing a serious amount of water to overcome the resistance inherent in the water to produce the same sound as air produces. So a shark wouldn't be able to play the harmonica. For that matter it wouldn't be able to play any reed or brass instrument.