Cooking water spinach safely
Water spinach, like other delta-grown crops, is host to a variety of parasites that can infect humans.  You should wash it in several changes of water; additionally some of my cookbooks (such as Asian Greens) recommend splitting the hollow stems to make sure they are clean on the inside.  Some sources recommend soaking it in salt water as well.

After that, it makes sense to blanch or steam water spinach for a couple of reasons; it kills any remaining parasites, and when frying raw water spinach without blanching, the stems tend not to cook all the way through.