Hi Salvo, I'm not a cold water swimmer, but I do get confronted with the issue on a regular basis, so I'll throw in some anecdotal information on this subject.
First, there are two factors at play here: water temperature and air temperature. The outdoor 50 m pool I swim in just outside of Munich, when I am on vacation, opens its season sometime in late April or early May. The pool is supposedly heated to 23 C, but in years past it was much colder than this. The Germans seem to be a pretty hardy folk and I would see, not only tri-athletes and competitive swimmers, but also old folk who do a slow breast stroke up and down the pool in conditions where I started experiencing hypothermia after about 30 minutes of swimming continuous laps. The worst part of this for me was the walk (about 150 m) from the pool to the shower after I got out. Somehow this year I finally figured out that I should go right up to pool side in warm clothing and, when I get out, I should immediately dry off and put on warm clothing for the walk back to the showers. Makes a really big difference.
I ask some of these people how they manage to deal with it and the Germans seem to like to appeal to cultural explanations for this stuff. They claim the people in northern Europe are more used to cold water (including the English) and they take it in stride. The mediterranean folk, according to this explanation just aren't used to it. I'm not mediterranean but I am skinny, but who knows? 55-60 F is too cold for me, and I won't last long in it.