I was watching a YouTube video titled "Fallout 4 Theory: Ghoul Whale" that shows him decked out in a full underwater outfit. Where can this be found?

It's a hazmat suit. It's not scuba equipment, but it does give you 1000 points radiation resistance (which is substantial and decent enough to swim for extended periods). Off the top of my head, there's several chem cookers wearing them in the secret room at Four Leaf fishpacking. I've seen them around a few other highly irradiated places.

Aha! That makes sense. And, I already have the hazmat suit. Thanks for the info.

No problem. Have fun exploring.