I have done that many times. I find that it is much easier to take a much larger amount of water into my body that way. Also, if I make the water in the tub very warm, almost hot, but not to hot, I can take water in my body that is much hotter than I can take otherwise.
The only things you have to be really careful of are: 1.) Do not make the water too hot that you will scold your body either inside or outside, 2.) Lay there perfectly still and let the water flow in at its own pace. You will be able to take much more into your body that way than you really think you can. 3.) If you think you to quickly let out the wate while you are laying in the tub, use an inflatable nozzle, preferably one of the double balloon type to make it impossible for you to do that. That is unless you also want to bath in what comes out of your butt when the water comes out.
I find that normally if I am either taking or getting an enema when I am not in the tub, the maximum I can take before it gets to be too much and forces me to have to throw up from it is about 5 quarts. However, if I am sitting in a tub of water the same temperature as what is going into me I can manage to get 6 quarts into me without throwing up.
I was able to get about 7½ quarts into me at one time without throwing up from it. This was done to me by a male/female couple I once went to for enema punishment to me. They had me lay in the tub flat on my back so I could look up and see the bag empty as it went into me. The bag was very large and held a total of 8 quarts. Yes, they did give me the full 8 quarts. As for throwing up, I had to do that in the water that was in the tub. In addition, when the bag was empty, they quickly deflated the two balloons on the nozzle they put in me. YES, you guessed right if you way that it was then coming out of both ends and I had to bath in both. Fortunately, they allowed me to get in a shower and wash myself off good after they drained the water in the tub and made me clean the tub when the enema was out of me from both ends.
There was a problem with the room smelling bad, but they solved that by spraying a rather generous amount of air freshener in the room when I started throwing up and while it was coming out of my butt. They also sprayed some of that air freshener in the water that was in the tub and on me.
No, I did not really enjoy that at all. I never went back to that couple for enemas ever again.