is July or August a good time to visit Dalian Golden Pebble Beach?

Yes, it’s a good time!
It’s sunny and a little hot during that period, from 25 to 30C. So you may bring your t-shirts, shorts, short shorts and sunglass.
I think it may be crowded, since it’s during summer holiday.
It’s about 270km southwest of the border with North Korea.

Middle of June a better time to visit?
Is it humid and rainy?

Actually, May-October is the best time to visit the beach, so it’s also great to enjoy there in middle June. According to former weather data, it’s not rainy and humid, and sunny in most days.