I thought that was just me, so it's interesting to see tangible evidence that it happens to other people, too.
Foot cramps are actually quite common during swimming, especially when you don't swim often. There's probably a study on it somewhere, but I suspect it has to do with foot/ankle position and the water resistance being something you're not used to. On the bright side, I usually stop having them within a couple weeks of consistent swimming, and I would think that the kicking motion is actually quite good for building/maintaining ankle strength and flexibility (but that might just be because my parents always made me do more kicking drills to strengthen my ankles for ice skating when I was a kid).
At least they didn't strike today (might have with a little longer swim, but still, I'll take it!)... and good to hear it helps with ankle strength probably - will want strong ankles for when I can run again, at least!
I assume it's to do with the angle of the foot, as I sometimes also get them when lying on the bench during massages.