Dwarven baths are a constant evolving project of mine, since I usually end up losing my forts due to syndrome (I've since learned to pretty much stay away from FBs).
I suspect you might be thinking about baths in the wrong way. If you're wanting the dwarves to clean themselves to prevent syndrome propagation, then having a spot where the dwarves have to voluntarily go to and clean themselves off is not the way to go. You want the dwarves to pass through a "cleaning station" routinely, and automatically, without any other actions on the part of the dwarves.
For instance, assume you have a single path to "the great outdoors", and you want to make certain that none of the dwarves track in any FUN stuff from outside. What you do is have a channel cutting across that path and have the channel filled with a slowly flowing trickle of water. Since the water is flowing, any contaminates get "pushed" towards the outflow so you don't get cross contamination by a dwarf walking through contaminated water. No soap needed. No voluntary baths needed. And nothing from the outside that hasn't been washed gets inside. And as a final little twist (just to make sure ALL the dwarves get cleaned), make the primary meeting area for the dwarves on the "outside" side of the watch channel. So when the dwarves go on break, or hang out in the meeting area, they trudge though the "bathtub" going to the meeting area, and upon their return, trudge back through the same bathtub.
Now, how to maintain that trickle of flowing water?
In the following diagram, pressurized water is supplied from the left, waste water flows away from the right.
########## ########
~~~~#┼^~~~~~~~~~~~~~~╬^¢##
####~##### ########
########## ########
In sequence from left to right.
1. Pressurized water.
2. A diagonal pressure reducer.
3. A door.
4. A pressure plate activated by water linked to the door. Min 0/7, Max 2/7
5. slowly flowing water.
6. A fortification.
7. A pressure plate activated by water linked to the hatch cover Min 2/7, Max 7/7
8. A hatch cover.
I'm using doors and hatch covers instead of floodgates due to their rapid response time. The 100 tick "off" delay on the pressure plate is bad enough without adding an extra 100 ticks to handle a floodgate.
What is not listed is a lever also linked to the door to start things up. The pressure plate upon installation immediately goes active since there's 0/7 water, but that activation status isn't passed onto the door with the door is linked to the pressure plate. So you gotta give it a kick in the pants and open the door manually.
What will happen with this setup is that the door will be opened when the water is below 3/7, and you'll get an immediate flow of very slow motion water. Then the door closes after about a 100 ticks and the released slug of water will slowly migrate to the right until it causes the water level to excess the trip point of the exit pressure plate, which in turn, opens the hatch cover to drain the excess water. With proper tuning, the channel the dwarves are forced to pass through on a regular basis will maintain a water level between 2/7 and 3/7 at all times.