> I have had this feeling twice. Both times my overwhelming thought was "Wow, I've been an idiot."

Same experience here. I'm a beginner swimmer and got caught in a rip tide in Bali.

The thing was, it was terrifying yet so calm at the same time. Everybody on the beach continued to bathe, unaware, and a few metres away I was frantically but silently fighting for my life.

The helplessness is especially haunting. You exert this primal will to life, and the force of nature just brushes it aside.

Anyway, I made it out, spent that evening binging Youtube videos about rip tides, and have developed a healthy aversion to ocean water.