Thanks so much for your insightful comment.
I agree swimming in lakes is scarier than the ocean. Although it is rather freaky jumping into the middle of the ocean which I've done while diving a few times.
Generally anywhere with a rainy season or hurricane season (Japan has both) is best avoided during that time. I got lucky and didn't get much rain this time, but you can't see the mountains in anycase. I've actually been to Jeju island which is in Korea several times and only seen that mountain once when I went in late spring. It's pretty random, but I think there are better seasons for viewing.
People tend to think of forests as natural, but the majority of them have been replanted. It means the forresters are good and planting large seedlings when you see them all in rows like this. It allows the most trees for the smallestspace I guess. It also looks very neat.