You aren't dehydrating yourself when you take an enema, you're leaching the salt from your body, which is no less dangerous. Water straight from the faucet has no salt in it, unlike water that took the long route to get there, so the salt in your body flows into it. If you're planning on holding an enema for an extended period of time or taking frequent enemas, whip yourself up a batch of saline and use that instead of tap.

The other two big dangers when taking enemas are temperature and water pressure. Your insides scald far more easily than your outsides, so the absolute hottest you'll want to have the water is a few degrees above body temperature. Too cold is safer than too hot, even if it isn't nearly as comfortable. Pressure is the big amateur mistake. You want to keep the bag only about two feet or so above the nozzle, any more and you risk tearing something. Some first timers expect to feel more of an effect sooner, so they raise the bag higher to increase flow. Don't. Have some patience. By the time you have the first quart inside you, you'll definitely feel it. If you feel like the water's coming in too quickly, take the bag down, wait until you feel better, and slowly take it back up. And, for chrissakes, do not EVER attempt to take an enema while inverted.

Keep these in mind, and you should be fine.