Almost Korea’s beach is open on 1st July.
Sokcho Beach : 1 July
Gyeongpodae Beach : 1 July
Daecheon Beach : 3 July
Eurwangni Beach : 1 July
Haeundae Beach : 1 July
Jungmun Beach : Late June
Actually almost Korea’s beach open all the time, you can visit , walk on sand at beach, but not suppose to swim.
During open period of beach, you can swim and enjoy sea sports.
I guess beach will be close bottom of August.
If you need more a detail info, Visit