Perhaps there is a difference in protocol depending on the number of treatments, methodology (proton vs traditional radiation aka photon radiation) and the location of your treatment. (For example: I believe there is some difference btw Mayo MN and Mayo AZ; both do proton beam treatment.)
I have detailed my experience here: https://connect.mayoclinic.org/discussion/rising-psa-2years-after-radical-prostatectomy/?pg=4#comment-708134
Briefly, I had five Proton Beam treatments (over two weeks) in Feb 2020 at the Mayo Clinic in Rochester MN. I had the SpaceOar spacer and fiducial (carbon?) marker placement about a week before treatment.
I had an enema before each treatment at an area in Mayo focused on that. I didn't find it embarrassing, nor did I have to insert the enema prep myself (a friend at home getting 25 traditional xray treatments had to handle his own enema each day - it wasn't always easy or 'fun'). I don't remember any requirement to drink a large quantity of water. There was no insertion of anything into my rectum (for treatment).

Not all facilities are equal or take the same approach for the same type of therapy. Loma Linda is one of the oldest hospitals providing proton beam treatment. I think the method of delivery may have changed but I believe they still use a 25 treatment protocol. It may also be worth saying that not all facilities charge the same amount for (nearly) identical therapies. Its my casual impression that Mayo MN may charge very low amounts for Proton Beam Therapy - maybe due in part to donors with deep pockets.