Welcome to the diving community mstrdm!
I believe that Deepstation is suitable for your needs, but you can't just show up there without any certification and dive. Your first experience will need to include an instructor as safety should always be the #1 concern when you scuba dive (see options below.) I did dive with one of Deepstation's instructors as a guide and he did speak some English, but I can't really comment on taking a class or getting certified through them.
The good news is that you have several options.
If you're not 100% committed to getting your Open Water certification, you can take
Discover Scuba Diving, which is a personal preliminary theory class plus diving experience with an instructor that allows you to 'try before you buy'. It's pretty basic and familiarizes you with the gear and allows you to do a shallow dive to see whether it's for you or not.
OTOH, if you decide you're ready, you can get your
Open Water certification (the entry level cert) by taking a theory course and then doing several certification dives, which give you basic cert. to dive to 18m with a buddy, rent gear and legit call yourself a scuba diver.
There is another post here in the Korea forum that tries to give the most up-to-date info on English language options here in Korea. Currently, your best English option is probably
Expat Divers in Korea as they just went full-fledged business recently and specialize in English instruction. They now have their own website. Contact Tim and the team through their
website,
email,
FB page, or Messenger. Tim is the Course Director there. I have dived Deepstation with him and several of his staff. They can definitely help you.
Best of luck! I hope you find something that fits for you! If you have any further questions, don't hesitate to ask.
