Assuming you use the same amount of soap and what nots, and get the same amount of dirt and debris off your body, the more water you use during a shower, the easier it is to process the water at your sewage treatment plant, if your waste water is treated.

If your waste water isn't treated, and is discharged to water ways as-is, the more water you use, the more dilute your pollution.

If you've got a septic system, I dunno? Probably doesn't help, but if your system is well sized, no big deal? Some of your outflow probably recharges aquifers, so it's kind of circular (although a lot of the outflow evaporates, so less directly circular there)