Snoek, I think snorkel position is a factor in your case. When you are standing on shore, snorkel must be tilted back, almost parallel to mask strap, to behind your ear. You are on surface face down, therefore snorkel will be out of the water. And, like mentioned above, clearing it with each breath should be automatic reflex. You are right about spooking the fish, I can see scools of perch dashing each time I blow hard, I think it mimics the sound of seals.