Discreet?

I have one like this
link
And it is going to be noticeable if you're only wearing tights or if you're in a bathing suit, but otherwise it's small enough!

It's hard to notice if someone is wearing jeans or anything not too form-fitting, but most people use this under dresses, skirts, or other loose(ish) fitting pieces.

Not 100% discreet, but discreet enough...

It's not as large as it seems, once in place it's surprisingly unnoticeable. Just don't wear tights and you're good!