I do this from time to time. It takes a bit of planning to have real fun. First, I made an adapter to fit on the end of the shower hose in place of the shower head. This has a male garden hose quick connector on its business end. As previous posters have noted, it's best not to do this as the first enema of a series, because it can get quite messy!
Assuming I am not that full of shit (some of my friends might not agree!), I first lube my trusty home made inflatable nozzle, and ease its 180 mm (7") length up my bum, seating the valve stem against my perineum before pumping it up slowly. That feels great and I find the pumping up is more than a little arousing! The nozzle is a separate unit, not permanently connected to any hose. It is designed to fill my rectum completely. It is fitted with a female non-return garden hose quick connector. I can walk around with it in place and there is NO danger it will fall out! Then I run the shower until I have a steady stream (not too fast) at a comfortable temperature, and simply plug myself into the shower hose with its adaptor. I can stand, sit or lie down if I wish - there's plenty of hose. Any time a cramp hits, I can quickly close the shower mixer control, wait until the cramp passes and turn the water back on again without upsetting the temperature setting. As my belly swells, I can unplug and go and look at the result in the mirror without leaking a drop. If I feel I can take a little more (or even a lot!), I can plug myself back into the adapter and go for it!
We don't actually have a bathroom scale at present, but you can see how easy it would be to weigh myself (full and empty) using this system.
Holding (assuming I want to) is no problem, for the inflatable nozzle can be pumped up to hold a full litre of air, and it takes a very great effort to expel it. When the time comes to expel, I deflate the nozzle where I stand and just let go on the shower floor. We have a wet room bathroom design that is very easy to flush down. I have another hower hose extension for that!
There's a challenge for the engineers among us to duplicate or refine my system!