Something thing that occasionally happens with cacti in the summer months is water splitting. This appears to occur mostly when cacti that are experiencing extreme summer heat and dryness get a sudden influx of moisture, which can come from rain, irrigation, or a combination of both. The dryness that preceded the watering causes the cells of the stem to shrink as the plants use the stored moisture for survival, which is of course the entire purpose of succulence to begin with. But the shrinkage can reduce elasticity in the epidermal skin cells, so that when water arrives again, the cactus absorbs too much, too fast, and the skin cannot expand quickly enough and splits open.
This senita cactus (Lophocereus schottii) just experienced two water splits this past week. One occurred a few days ago and the other occurred mere hours before this photo was taken. The senita is a rooted cutting that has been planted for about 3 or 4 years, and stands about 3 feet/1 meter tall.
To my knowledge it is the stem-succulent cactus family that is by far the most prone to doing this, as opposed to leaf succulents such as agaves. Larger cacti are more likely to experience it as well, as opposed to smaller species, and the splitting usually occurs in the middle of the stem between ribs rather than up near the areoles. The water splits shown here are on a senita cactus (Lophocereus schottii), with the left fissure having just opened this morning – it was still moist inside at the time the photo was taken – and the right fissure having cracked a few days ago since it’s had a chance to scab over and turn tan.
Close-up view of a fresh split, which must have opened sometime in the overnight hours since it was still green and moist inside at sunrise. Cacti quickly scab over exposed tissues, usually within a day or two (as seen in the older, tan split to the right), so the freshness indicated a very recent event.
Water splitting is generally not harmful to the plants in the long run, although obviously it is aesthetically undesirable. They will heal up and become part of the character of the plant, but they will also never go away and are permanent. Plants that are kept well-watered during hot dry spells will probably not experience water splits, and of course many plants that are not watered extra won’t either. It occurs on both wild and cultivated landscape plants, generally ones outdoors in full sun. There is a slight risk of pathogens entering the split, but I believe it’s generally a minimal health risk since cacti typically deal with wounded tissues by callusing them over within a day or two. If you ever see this type of long, narrow crack or a lenticular-shaped scar in between the ribs of a large cactus, then you can be pretty sure that it was water splitting at some point in the past. 
Mine is probably from over watering. Do I stop watering it daily and let it dry up a bit? The split is horizontal and deep. Will it survive?
No cactus requires daily watering, so if that’s been happening they scale back. I have no idea whether you did this watering via automated system or by hose, but daily is too often. I also do not know the species you have, and tolerances of wetness vary to some extent with the species in question too. Generally speaking once every 1 to 4 weeks is enough as long as when you do it it’s also fairly deep and thorough. Again, the species and size of the plant dictates this, and I can’t offer more specifics without knowing what species you have or where you live.
Saguaros and most other large cacti do best if you water deeply and offer a soaking amount of 10-50 gallons or so, depending upon size and recent weather and temps, and then wait a few weeks before repeating. This is in summer – winter is less or none, especially if it’s been raining like it does. But if you have a water split and there’s no sign of rot, decay, or infection then it’s probably fine. The split is a permanent scar and will never go away however, although it shouldn’t harm the plant’s health in the long run either.