Interesting activity to research. It appears bag of ice applied to your face for not more than 30 seconds can set off a reflex activity to bring your heart rate down. However, as any Spanish lifeguard can tell you, If you lie in the sun and roast gently for half an hour and then dive into the sea, the same response can kick off a heart attack, reduce your ability to swim and cause you to drown.
mammalian diving response: Interesting... - AF Association
Divers response is well known, but it is different to cold water shock which can be very dangerous - a local RNLI crew died a few years ago from cold water shock.
(I used to do a lot of scuba and my husband worked in the professional diving industry for many years). I always used to have a reflex action as I put my head under water - every time - even after hundreds of dives. I just had to calm myself and once I was down a couple of metres - I was fine. Taking a breath in, holding your nose and breathing out very slowly against resistance has the same effect - vasovagal-vagal response. It can flip me into NSR but doesn’t always work.
I posted a pic today of bathing in the sea here in East Devon, but I took my time going in whereas my 84 year old husband dives straight in and thinks I am mad walking in! Cold water bathing is a great way of improving vagal tone, which over time, will help. I feel great after!