Here's some info from the Arizona-Sonora Desert Museum in Tucson, AZ.
http://www.desertmuseum.org/books/echinocactus.html

'One of the great fables of desert survival is that barrels and other cacti are reservoirs of water that can be easily tapped and drunk. It is true that indigenous peoples and a few other desert residents know how to obtain emergency water from cacti. But most city dwellers, including most aspiring survivalists, could not get water from any cactus if their lives depended on it (pun intended).

'The first problem is getting to the pulp inside the very tough and spiny epidermis. A pocket knife is inadequate, and tools that are typically carried in a car, such as tire irons, aren’t very effective either. The labor of cutting into a barrel on a hot day is likely to cause loss of more water from sweating (and perhaps bleeding) than one would gain from the cactus.

'Secondly, the water in cactus pulp is tightly bound in a gooey mucilage. Most of the year the pulp is more like a damp sponge than a watermelon—you can’t squeeze much liquid out of it. Furthermore, the raw pulp of many cacti is inedible. Some species have potentially toxic levels of oxalic acid (prickly pears), bitter and sometimes toxic alkaloids (senita and many other cacti), or other substances that cause diarrhea (some barrels) or vomiting. The best cactus for emergency water is Ferocactus wislizeni. See more details under that species. For a historical account of extracting water from a barrel cactus (in cool weather and when it was legal), see Camp-Fires on Desert and Lava by William T. Hornaday, pages 216-219.

'Water can be obtained from cacti using a machete and solar still. But anyone with the foresight to pack these tools is smart enough to carry plenty of water and inform friends of the itinerary and expected return date!'

Sue