I've gone cliff jumping at Saint Mary's Glacier several times, and the cold shock is real. Even with all the adrenaline, hitting the water punches the breath right out. Once the spasms pass, it's really not too bad, and honestly quite fun.
That being said, if you were a weak swimmer or prone to panic, it could go very badly very quickly. Lots of respect to a guy doing it in essentially open ocean, with the added difficulty staying on course and extreme variability in conditions.
That being said, if you were a weak swimmer or prone to panic, it could go very badly very quickly. Lots of respect to a guy doing it in essentially open ocean, with the added difficulty staying on course and extreme variability in conditions.