I have an outdoor hot tub which I use on an almost daily basis. It is mostly cleaned with ozone, but does require the occasional dose of chlorine. Whenever it does, I make sure that I am not in it, and that I won't be going in it for awhile. It still worries me, but I just love using it, so don't know how to get around that one.
I have always wanted to use Epsom salts it it, but from the little bit of info I could find online, they recommend against it. I did hear about one guy who had been using Epsom salts in his outdoor spa for many years and raved about how good it was. But other people warn that it will destroy the equipment and void any warrantee.