What scares you beneath the surface?
Early on, I would have to say other divers -- and was once told that, should someone be kicking you in the face during the course of your training, that same he or she will be doing that when you hit the ocean.
That turned out to be true.
Not so much fear as considerable irritation, I once lost a rental Oceanic Ocean Pro dive light -- a massive thing with an molded handle, to a klepto Giant Pacific Octopus, at the Tacoma Narrows, back in the mid-1990s, which started just as a slow tugging in the dark; an unpleasant notion of entanglement (which is creepy enough by day -- far worse at night); and then a sudden, swirling mass of red and white flesh, all the while experiencing the swift currents for which that site is well known.
Think of the "face-hugger" from Alien but much larger.
The light was attached by a jerry-rigged length of gangion which came undone from my BC; and the critter retreated to its shell-strewn warren -- the light still arcing back and forth in its arms -- in about fifteen meters of water, among ruins of the original bridge (aka “Galloping Gertie”), which famously collapsed in 1940 due to what they are now referring to as "aeroelastic fluttering."
My dive partner, a friend from the Gig Harbor area, and familiar with the site and its denizens, dissuaded me from attempting to retrieve the light (for which the local shop charged later me 100.00), from what he later estimated to be a 20 kilo octopus (he once did species counts and weigh-ins) -- and there it remained . . .