Cramping Question?

Never had this problem before, but for the last couple months, as soon as I pass 800 meters in my main set in the pool, I am starting to get calf cramps. I have tried stretching before, have tried eating more salt lately, drinking more water, but keeps happening at about the same time period. Any thoughts / help would be much appreciated! Stretch more, get those salt tablets, whatever works for you would be great to hear!

Thanks