There are actually a lot of dive centres around Ulsan, Busan and Korea as a whole, just take a look at the store finder on padi.com also ssi, naui, bsac and other agencies have strong presence there.
As above the issue is the language barrier, I work as an instructor for a centre in Busan and am the only (native) English speaking instructor in the area to my knowledge. The nearest to me apart from the shark tank (Busan aquarium) is in Seoul, Jeju or Oosan. Some of the Korean outfits do have English ability but it depends on how much you actually require.
Shirley - it sounds like the store you saw on the sea front was in Haeundae... as this is the prime beach front area of the city there are limited SCUBA activities there. diving from that point is almost non existent as it is very featureless and the overheads are so high, the most of the dive centres in Busan are located near a beach called Taejongdae which is on an island from the south of the city called Yeongdo
Cool, if wither of you are ever in the area and want info on what is going on then just drop me a line