I will have a trip by the end of this month to a northern country to "escape the heat" where I live in, and I would definitely want to do some swimming at some best beaches there. The expected water temperature would be around 5°C (41°F).
However, as I live in a hot climate, thus "escaping the heat" on the trip), 5°C (41°F) is a big gap from the temperature that I regularly swim in. This year I have swum in water down to 18°C (64°F) well, but the weather in my place hasn't gotten any colder than that this year so I don't know what is my lowest comfortable limit (I am aiming at 15°C (59°F).
In the past I had swum in water down to 13°C (55°F), but at that time I was not serious about cold water swimming. I didn't have any knowledge about hypothermia, and I didn't take my time. I just got out when I felt that my swim form was deteriorating. I think I have encountered the effect of afterdrop once, I no longer know when, how long and what temperature it was at that time as I didn't have knowledge on hypo. At the moment what I knew it was that, I got out when I could no longer hold the form, then ran to the shower room and turned on the hot shower. I still found myself shivering even more vigorously and started fainting, so I turned the shower temperature down to reduce the blood circulation. I only stopped shivering after a relatively long time before I turned the temperature up again. Now I know I was experiencing the effect of afterdrop at that time, and I shouldn't be taking a hot shower at that time, but I don't have the numbers on hand.
So how should I feel when swimming in 5°C (41°F) water, and how should I properly plan my swim and my exit strategy, as the difference to what I am used to is too large for me to imagine?