The ONLY painless one I've found is using my husband's urine, fresh of course. He doesn't drink or take any meds, and is free of disease. You can use your own, it's sterile. It may or may not be for you, but I got over any fear of urine long ago when my exhusband surprised me by turning on his hose in my butt. I realized that not only was it not a huge pain, bags, temp, hoses, clamps, but not a physical pain as well.
The closest you can get to the body's natural water is second best, purified water, NOT TAP!!!, with salt. I forget the ratio, I think it was one teaspoon per quart, but I'm sure it's on here somewhere. I HATE cramping. I've had plenty of natural cramps having Crohns disease, I avoid anything that might even think about giving me those type of cramps. So believe me when I say, that for me, the urine has been painless and enjoyable. It's the only way I can enjoy an enema during a disease flare up. The only thing that finally motivates me to get up and go to the bathroom after doing one is not wanting it to move to far into my colon, no pain or sense of urgency.
Also, you may want to inject a small amount of coconut oil, use the good stuff if you can afford it, prior to any fluid. You don't want to retain this too long, or it'll move further than you might want it to. It helps protect the lining of the intestine and also coats it which lessens the likelihood of fluid being absorbed by the colon. I use 2 oz for anal sex prep, and 4-8 oz for enema play.