Went to Ghanyeon recently and you can get in plenty of climbing in the mornings and late afternoon. And the river is GREAT for an afternoon swim especially when it's hot! However, the "lifeguards" will blow their whistles at you if you go in too "deep"
Don't know where you're going in Seoraksan but the road to Aghulbawi may still be out so the crag could be inaccessible but if you're going to Ulsanbawi- this won't be a problem. Maybe some of the Sok'cho folks have some info?
Have a great time where ever you end up!