The field of what might cause cramps is quite contested with a lot of controversy around the heat/dehydration cramps, but I find it surprising that no specific theories appear to have been proposed (or tested) while swimming, usually in cold[er] water. There's one governmental site that does acknowledge cold and cold water in particular as a risk factor. I for one never get cramps in a pool/water over 18 degree C, but I am quite likely to get them below that, no matter what I do in terms of stretching, drinking etc. (There's one other site that mentions that "muscle spasms" occur during cold-water shock, below 15 degreed C.) Am I correct that no specific theories have been proposed or tested for cold-related cramps?
Interestingly, the cutoff temperature I found experimentally for myself is perhaps reflected in one article:
during swimming at a water temperature of 18 [deg] C, it is noted that esophageal and muscular temperatures tend to decrease, below 37 [deg] C. [3 refs cited for this fact]
So below 18 C water temperature it appears that consistent cooling of the muscles occurs. So I guess one could ask, equivalently, why muscle cooling [below 37 C] can lead to cramps.