https://www.epa.gov/watersense/data-and-information-used-watersense

WaterSense annually estimates the average cost of water and wastewater in the U.S. based on an analysis of data from rate surveys conducted by the American Water Works Association (AWWA)

For 2023, WaterSense’s estimate for average national rates are:

Residential cost per 1,000 gallons Water: $6.64

That's $0.006/gallon.

All showerheads installed since 1992 have a federal maximum of 2.5 gallons/minute. Some states have lower limits. Assuming the shower is on at maximum flow and is a showerhead that can do the federal maximum, that's 1.5 cents a minute at maximum flow, or 90 cents an hour. You'd have to have some pretty prodigious stamina to make that a substantial cost.

I'd also note that one could reduce the cost by using a low-flow showerhead or reduce the per-capita cost by participating in group shower sex.