Good that you understand that.
This will DRASTICALLY reduce the boats that qualify. A separate shower is relatively rare on boats in your size range. You can probably find at least 10 different boat models without this for every one that has it.
I would ask if this is really an absolute "must have." Maybe you should charter a boat that has a combined head/shower and see if you really cannot live with that arrangement.
A good place to find a lot of particulars on a lot of different boats is sailboatdata.com. You can start looking through various boats there, and compare the layouts, as well as almost all of the basic design parameters.
Good luck.