Did you know that many people drown because of cramps when having a swim in deep waters? You've all had cramps, I'm pretty sure. It's when the muscles (mostly it's the calf) suddenly become so tense it hurts awfully and you can't move your leg (or any other part of body). It happens as a result of big physical tension or sometimes sudden temperature change (for example cold water, it's colder where it's deeper). Anyhow, if you ever get these spasms in water the main thing is...
1) of course not to panic (that will do no good)
2) while keeping your body on surface of the water with arms, use your healthy leg to stretch your invalid foot, try to pull it with one of your hands getting a grip on your big toe.
3) it will help if you guide the pain signal somewhere else (sounds weird): pinch or even bite yourself. The best would be having a tiny safety pin on your bading suit, then you can use it to prick yourself into the crampy foot or elsewhere - it should help the spasms.
4) while swimming back to the shore keep taking short rests on the surface of the water.

That's scary, isn't it? Did anyone have that experience in the water? I get cramps once in a while but never when beeing somewhere deep. I had a bad one in gym once. Turned out that I put extra loads on my legs that were of different weight. The foot that had a heavier load was quite upset with me for a while, I should say... :)