Lovlee, your concern is understandable, as is your excitment. May I suggest that you take a small enema first and expel before playing with your partner. It's always a good idea to protect the bed with a waterproof crib sheet and/ or absorbent pad, easily purchased from any drug store. An inexpensive shower curtain-liner works equally as well and can be discarded afterwards.
Furthermore, there are several techniques that you can try. One is to use a long, soft rectal or colon tube. This not only gets the solution up past the rectum and sigmoid colon where it is more easily retained, but is soft enough to prevent injury should you or your partner become overly excited. Finally, a warm wash cloth pressed up agains the anal opening following the enema is usually sufficient to keep the water in.
Some individuals will recommend inflatable nozzles "very expensive" or inserting a large but plug "often painful", neither of which in my opinion completely prevent leakage. Just go slow, have fun, and remember.....it's only water, so a small amount of leakage, providing you protect the bed is no big deal.
Even accidents can be fun. I remember years ago lying over my lovers lap while she gave me a soapsuds enema. Just as I was reaching my limit, she exclaimed that the clip broke and that she could not shut off the flow. In a panic, my friend pulled the still spurting nozzle out of my bottom flooding both of us with warm soapsuds. I still recall the feeling of my erect member sliding against her slippery soapy, thighs, and climaxing right there over her lap amidst our laughing and yelling....Ah to be young again....