I like expelling in the tub. I could give a hundred 'reasons' for it, but they would be dishonest. I like it! Why, then? It feels deliciously abandoned, mainly. That's sexy. I just let go when I'm ready. I get dirty - so what? I'm naked and in the tub, aren't I. Tell me a better place to clean up, somebody?
Usually I'm plugged with a monstrous home made inflatable nozzle (it's over three inches wide inside me), but even that will leak when holding a big, strong, irritating enema (say, 5 litres) for a quarter of an hour. No problem in the tub. Leak away! If I get cold, turn on some more hot water. Read a book while waiting for the cramps and the gushes. I feel free to squeeze my bum cheeks together for the pleasure of the sensation of waste and enema water forcing its way through anyway. I guess that's infantile, right? OK, I still like it.
In a tub enclosure with curtains or shields, I can change positions at will without worrying about squirts from the effort of moving (happens!). Knee chest is best to get rid of farts, and best done early in the expulsion. That DOES splash. Belly down feels particularly helpless and nice, but squirts can go high! Lying propped up on my back is the height of luxury, but not the best for getting everything out.
The tub is a great place to expel, and in the bathroom I use for my play, the toilet is right next to it. Getting most of the crap into the crapper is no big deal, and what's left goes easily down the drain of the tub. Yes, the tub gets my vote!
Johnny