I'd be careful as it's not exactly "legal" like the soaplands are in Tokyo. Also tough if you're a foreigner.
I haven't had any luck as a foreigner in Seoul, but did have a good time in Tokyo.
I'd recommend a H/S or HJ spot first before jumping straight into it, but Great Pharaoh might suit you well for a first-timer, no fluid exchange, so it's clean and professional.
I'm surprised no one has mentioned the episode of Seinfeld yet where George pretends to be an out-of-towner to date the tour guide, haha.
Anyway, good luck with your search, OP.
My best friend actually introduced me to the hobby years ago, other than that, no one knows lol (I'm a single guy, so no secrets anyway, I could walk into the non-discrete Bamboo with no issues)