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I believe the different muscles being used are in the lower back, glutes and hamstrings in that they are used in a different config than while on land and your toes are plantarflexed the entire time. Then when that entire posterior chain puts more pressure on the nerves that feed the calf already in the "about to cramp position" you get the camp. If your foot is entirely dorsiflexed and calf stretched, it is harder to cramp. When a soccer player cramps the trainer pushes on the bottom of the foot and shortens the shin muscles and elongates the calf. In swimming you're in the extreme opposite scenario. At least in pool swimming you go to a dorsiflex at wall push off. Open water, it's 100% plantarflexed.

Your electrolytes being "within normal limits" is no indication that you wouldn't benefit from magnesium supplements. Those recommendations are averages meant to suggest what will keep most people in a population from sinking into chronic illness. They provide little guidance for an individual who is doing something that 98% of the rest of the population does not do, and they provide no guidance for athletic performance (vs. basic health).

I've encountered very few people for whom magnesium supplements don't provide at least some relief from cramping. They may or may not help you. But as others have said, you can probably also find some relief by tweaking your stroke.

I sometimes get cramps while swimming if it's the 2nd workout of the day, especially if the 1st was a hard bike or run. If it's swimming first I don't seem to cramp - muscle fatigue seems to have something to do with it for me.

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Or there may be simpler reasons. The salt water gives more buoyancy, so you kick differently than in fresh water. I'd also guess that the salt water you're in is generally colder than the fresh water, which may have an influence. And with fins, you definitely kick differently, regardless of the temperature and/or salinity.

I get leg (and sometimes whole body) cramping if I swim in the morning after a hard bike workout the afternoon/evening before.

Lately I have been making sure I take in lots of fluids during and after the bike workouts as well as an electrolyte supplement (saltstick or thermotab), and this seems to have resolved the problem at least over the last couple of weeks.

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As others have said I typically only get them in the pool. I think its the power of the push off and then having your toes pointed which is a pretty unnatural position. I was getting it bad a while ago and tried the pickle juice trick and it worked really well (the sci says it doesn't prevent but can cure). Most surprising thing that has ever worked.

LOL! I had a cramp in my calf today (never happened before), and I too, am blaming this thread!

Edit: had to pull myself to the end of the lane using the lane line for support, and get out onto the deck to stand and relieve the cramp. The lifeguard didn't seem to notice anything, at all. Unfortunately, the lifeguard is a dude. otherwise I would have asked for/demanded assistance.

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Not everything is as it seems -Mr. Miyagi

I've had the same issue and concluded it was simply dehydration combined with a strong push off the wall. That motion is different from running for biking so it works those muscles in a unique way.

Solution = hydrate & push less ( pushing off the wall strong will not make you a fast open water swimmer anyhow). It worked for me.

I used to get it as well, especially in long swims. But only when I stopped or switch from freestyle to breaststroke at the end of the swim for example. I did not get it while swimming, although I could feel it coming. I would feel some pain (discomfort/tense muscles) in my calf/ankle area while swimming. This was usually when I do more than 3K. I feel this was related to having my toes pointing to the back for a long period as I do when kicking, or maybe the way I kick. As some others mentioned, with me this got less as I swum more frequently. I started taking salt stick before long swims as well. This seemed like it helped. Although I cant really say if it was the salt sticks or the increase in frequency of swims which made the cramps occur less. When I dont swim for a period, and go back, I get them again.

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