Funnily enough I was looking at a website about this today, because I would really love to have this done. I found a link to a spa near me that offers this, and I think they cleanse your feet beforehand, so the water wouldn't have had grubby, manky feet in it, plus the water in the tanks goes through state of the art filters, is sterilised and the tanks have UV light filters too.
The spa I was looking at want you to fill in a health questionnaire before you have the treatment, and you have to tell them if you suffer from psoriasis, asthma, epilepsy, excema, heart disease/pacemaker, phlebitis/blood clots, dermatitis, diabetes, high blood pressure, hep a or b, HIV, or if you have an open wound on your feet. It doesn't mention verrucas, so maybe that would be OK.
I am going to wait until the split in my heel heals and then I will be making an appointment - I can't wait to have lovely soft feet, though with the amount of hard skin I have, it may take more than one go, apparently!