A guy that I work with gets cramps swimming, but I never have - sometimes side stitches if I do hard intervals, but that's usually telling me my breathing is getting bad and I need to relax.

Maybe hydration? Electrolytes? Potassium? You are still burning calories and need to stay hydrated even under water, your body temperature does rise... maybe in the case of the breastroking it's using muscles that aren't developed? I have been doing butterfly/back/breast in place of 3 sets of drills and boy do they hit me differently, but I can't say I've cramped.

Good mystery to solve - maybe what makes you cramp makes me slow and we could both benefit from the solution

Cramps in your toes!! OW!! I can't even imagine what that feels like, but I think of it as a kind of hitting your funny bone thing. Maybe pins and needles after your foot falls falls asleep.