Origin Of The Species

A question came up: what is the origin of the term "bubblehead" as a term of derision/endearment for Submariners? Is it because of the need to keep a "zero bubble" on the inclinometer? Because of the bubble-looking headgear worn by early divers? Or is there some other reason? (Bonus points to whoever can come up with the actual answer, accompanied by a citation from a reliable source. My Googling skillz weren't up to the task, which is fairly rare for me.)