If you get there, and there is no seat in the shower (there will be if you have reserved a roll-in shower room, cross my heart!) then you can call Housekeeping and ask for a seat to be brought around. In the spirit of full disclosure, those seats can be a bit.... well-loved and well-used, so you also have the option to just have
Amazon deliver one. You should be able to grab one for around $30, and just leave it there when you check out if you don't want to lug it home.
The biggest concern for me would be if it is the firm surface seat, or the "sling" style mesh seat. The mesh seat can be difficult for some folks because it has some "give" to it, and we had a member here who struggled with transfers with that style of seat. I don't like it for other reasons LOL. Having said that, we also have folks here who have no problems with it at all. Your mileage may vary...
It appears in the picture that our friend @landjudy posted that it is the firm surface seat, and I will bet you beignets at POFQ that it will fold up out of the way so that others who don't need it can stand directly under the shower.