OP
Darkwood
I would suggest liaising with a plumbing Engineer and have a site meeting with the clients and/or foreman if there is one, if this as you imply has already designed then query whether supply demand, availability etc has already been confirmed and if not highlight your concerns and ask for a site meeting with all relevant people... also find out average daily/weekly use and have ready a projected energy cost to run these showers, if as you say they be in use most of the day then this could be a big cost although i find it hard to believe they would be in use that much ...only the like of swimming pools on weekends see a constant flow of traffic through the showers.