Gosh, that's a new one @melanieg51
Containment of body fluids during chemo really is a 'thing' The drugs are nasty and can affect other people who have come in contact with them. Personally, I wouldn't want to be in a small tank of water after someone who may, or may not, be leaking cytotoxic fluids (via a variety of methods) had exited it. Nor would I be keen on getting into a body of warm water dense with other peoples detritus during chemo.
That said, I know very little about floatation tanks and what sort of risk management strategies they put in place. If it was fresh water for every 'floater' you'd think it would be OK. The business should have a policy on it and your oncologist may have an opinion too.