I was maybe 15/16, family holiday on the island of Menorca. There was a long cove where I would snorkel. Saw hundreds of sea urchins, some fish, an octopus here and there. Water was warm and clear

I surfaced and kept heading out. All of a sudden the water went from warm to cold. Goggles still on I looked down into the water. Darkness. Nothing.

I don't think I have ever swam faster back into the cove and got out.

Phobic? Probably not but it was very scary.