Hundreds of eyes
To get your diving certification, some places require you to perform a dive in pitch darkness. In this person’s case, that meant going underwater without a light source and trying to survive. Assuming you’re in a relatively calm part of the ocean, that doesn’t sound like much of a problem.
But what does sound problematic is seeing hundreds of eyes looking at you. Even if it wasn’t pitch black, we’d still be pretty unsettled by the glares of hundreds of fish who think we’re trespassing. But what was even freakier about the situation was that the OP had no idea what the eyes belonged to.