I got zapped by a big one, when I was about 12 years old.
Didn’t see it until I was about 3 feet away.
Huge, ugly thing, with tendrils flared everywhere. I panicked and tried to paddle away (I was on a rubber inner tube, of all things), but a wave pushed me right into the monster.
Hurt like hell to the 40th power. The pain was so intense that I wailed like a baby. The feeling, IIRC, was part high-voltage shock, part flash chemical burn. Left welts all over my legs.
Next time, I nuke it from orbit.