Towel". I've lived aboard sense 1989 with the exception of six months in 1993 and the last 2.5 years and I showered every day aboard. The secret is to use a standard towel on the floor just outside the shower door (or door to the head if they are the same)as a bath mat. After the shower and you have dried yourself off, pick up the towel and completely wipe down the inside of the shower/head. Top to bottom as well as the shower curtain and overhead. Do this every time without fail! Drain the sump after each use and about twice a month clean it out completely. Hang the towel out to dry change it every few days. That 23.5 years worth of showers and never a mildew issue. What can I say? Also, but not having to do with the shower issue, once a year I would take my "Hudson sprayer" that once doubled as my shower pump, mix up a batch of "Strike Bag" or water/bleach solution and and spring clean the entire boat. That meant dragging almost everything out onto the dock and doing a wipe down of the entire inside. Usually this took place after a particularly hard passage where things were wet anyway just from the moisture in the air and that occasional pesky leak. I would do the bilge as well as the underside of the decks and inside the cockpits and engine compartment. It was a lot of work once a year but like I said, 23.5 years is a pretty good run.