Calf cramping

I swim regularly. I swim a lot. In general, I rarely get calf cramps. (Or any cramps.) But this past weekend I did my first open water swim (2.4 miles) and well before the first mile I started getting cramps. First in one calf, then in the other. I did pretty well to concentrate on relaxing and keeping them from knotting up too bad, but it hampered my ability to swim as hard as I would have liked. (When I gave it a shot and tried picking up the pace, the cramps started getting worse, so I just kept the best pace I could and finished it out.) The OW swim was less than the distance I swim daily, so I can't attribute it to the distance. But the water was colder than I am used to. (I usually swim in a pool that has temps between 82-85.) Water temp was 72. My sister suggests that this was the cause. My wife thinks it's because I am used to swimming in a pool, and every 25 yards I get to stretch my calves on every push off. Another participant here in PM suggested I increase my potassium intake (bananas) but I am a regular (daily) banana consumer. (Maybe I need to eat even more?) I also take calcium supplements (and multivitamin and other things.) Without question, I am a weak kicker. I suppose I could work on strengthening my legs with more kicking workouts... Someone has suggested that I do calf raises on the edge of a step to strengthen my calves. I'm looking for ideas about what might have caused the cramps, and what I might do to prevent them in a future event like this. (PS: Would this have been more appropriate to post on the Open Water board?)