The shower head that I have I dont think would pose a problem to the inside of your body. However, If you are looking into doing anal sex often I would do as suggested above. But an enema from the store or an enema bag that you can reuse. There are plenty of gravity fed bags online fore less than 20 dollars.

In the past I have had a water enema and it felt pretty much the same as when you were about to have a bowel movement. It was just a full feeling, and plenty of time to get to the toilet. Just make sure to test the water temperature before you insert the water.