Possible and likely causes for cramps in runners are overexertion,   underhydration, poor circulation, and electrolyte imbalance.   Undernourishment is another, but I'm confident you know when you are   starving.

Exertion is the easiest to rule out because you  know  whether or not you are pushing too hard, and you described toes  cramping  as soon as you hit the water.

Hydration is a  umm, hot  topic this time of year, and easy to underestimate as we  dehydrate all  day in air conditioned cars, stores, offices, and homes.  Then we go  crazy and exercise, sweating out water faster than we can  possibly take  it in. A dehydrated body lowers blood volume, which leads  us to the  third possibility...

Circulation problems  often become  evident in the distal extremities of fingers and toes.  Cramping of these  though, involves muscles that may be several inches  away from the  digits in question. There are other smaller muscles both  under and over  the foot bones that can cramp and deform the foot  structure, something  I've experienced after marathons or long training  runs. I don't think I  was experiencing a circulation problem at the  time, so we'll move to the  last item...

Electrolyte  imbalance goes hand in hand  with hydration, since water loss through  the skin also takes with it the  vital mineral salts we call  electrolytes. Potassium is the poster child  for these, since it has to  be balanced internally (not in the diet) about 20x over sodium, of which the  average diet contains  way too much, but Potassium is not the only electrolyte people  tend to be  lacking in. Also important to muscle and heart function is  Magnesium,  which is deficient in roughly two-thirds of the population.  Dietary  Magnesium is present mostly in things like dark leafy greens and  beans,  which are not very high on most people's shopping lists.

While   many people pay attention to another important mineral Calcium, they   often forget that it should be balanced with about half the same amount   of Magnesium intake in order to insure proper muscle and heart function. This is true  whether or not Calcium is being taken as a  supplement. Most vitamin and  mineral supplements have to skimp on the  minerals because they are so  bulky and the pills can only be so large,  so don't depend on the  average multi to get you what you need of all the  important  electrolytes. A spinach salad and some bean burritos will be  better  insurance. BTW, Calcium tends to constipate, and Magnesium has the  opposite effect. Don't say I didn't warn you...

Of the above, I would say hydration  and electrolytes are the frequent  flyers. By all means monitor the  others, but stay on top of these two  every day.