I understand there is a single shower in the Amex Centurion Lounge. Holders of run-of-the-mill Amex card holders can access that lounge for USD 50. Its free for Centurion and Platinum Amex card holders.
I've read in the relevant thread over in the Amex forum, that there is often a lineup for the shower. :-( Why they would build this with a single shower I dunno.