Hyopjae beach is by far the most beautiful beach I saw while I was in Jeju. White sand, volcanic rocks, turquoise water and views of Biyangdo, a small island off the coast.
It was quite windy when we went here in mid-September so I didn't go in the water. It didn't stop the boyfriend from getting in, though.
The only people we found hanging around here were three or four small groups of foreigners. Mostly, Koreans came, took a photo and left. Which, with the wind as strong as it was, I totally understood. But, at least the sun was strong enough that you could feel comfortable when the wind let up for a minute or two.
I highly recommend checking out Hyopjae beach. It was my favorite beach (for views) in Jeju. You can reach it by bus like we did by getting out at the Hallim Park (한림공원) bus stop, just after Hallim City. While you're here, you can check out Halilm Park as well, it's right across the street.
For more Jeju information, check out all the articles in my Jeju Guide. In the next few days and weeks you can expect even more posts about Jeju.