The feeling of water running down your legs is part of peripheral neuropathy. I have that too, and have had it for 26 years. Many of us have that feeling along with tingliing, burning, zapping. We use medication to help control those feelings -- the medication was originally used for epilepsy patients, but they found that it worked well for neuropathic patients. i've been taking meds for that for many years. i do NOT have arachnoiditis.
I know of your concern. I think they probably would have seen it on the MRi if you had it.
God bless & take care. Hugs, Lee