Sounds like you went to a shitty place given the water quality you mention and also, that you didn't really try the experience as intended.
I'm real skeptical of stuff like this but did go try one in Vancouver a year or so ago at a real high end place. Everything was highly sterile and very comfortable. I tried to go in with a very open mind and it was intriguing.
It definitely took a few minutes to properly relax into floating so there was no muscle tension which creates the discomfort. Shutting your mind off is a real skill but I definitely managed to relax for awhile. I started experiencing a ton of light auras, all kinds of colours and it didn't matter if my eyes were open or closed, they didn't go away so I wasn't seeing them optically or whatever. Kind of surreal, super trippy.
I can see how it wouldn't do anything for lots of people and how it could for some. Personally my mind is always loaded, working in a constant high stress environment, I'm the type who wakes up 4 or 5 times a night thinking about projects, issues, etc. Finding a way to take an hour and actually shut down was of value to me but, like most things, it's mostly mental gymnastics. It's no miracle pod, but it did facilitate a way for me to relax in a manner I could buy into. Placebo effect? Probably. But of functional use for me.