Hamdeok is about half an hour east of Jeju City on the coastal bus, which makes it the beach most people reach first. The bay is shallow, sheltered by the headland, and the water sits at around 23–26°C through the season.
The official swimming season runs from roughly the first week of July to the end of August, when lifeguards are posted. Outside it the beach is free to use and considerably calmer.
Climb Seoubong before you leave. It takes twenty minutes, it is the only elevation for miles, and the view down onto the bay explains why this particular beach ends up on the front of so many Jeju guidebooks.