Ever had both calves cramp up on you in the water?
I used to, all the time. For about 20 years, swimming laps has been my main workout. If you get calf cramps, try this: before getting in the water, do a few seconds of stretching where you stand on the ledge of something -- a step, or the bench in the shower room, on your toes, with your heels and most of your feet hanging over the edge of the step or bench. Raise up on your tippy toes, and then lower down as far as you can on your toes, so that your heels are below the level of your toes, to gently stretch the calf muscle. Do it gently, slowly, about five times. A sports therapist taught me that trick, and it works. I swim quite a lot with a variety of fins (hence my screenname!), and as you know, fins will give you calf cramps but quick if you're not conditioned, and even sometimes if you are!
I still occasionally get calf cramps in the water (when I'm not getting enough potassium in my diet, I figure), and when that happens, I concentrate on RELAXING the muscles in both my ankles and calves. It takes real focus, but it also works.