Yue Tuo Island in Laoting, Tangshan is the cleanest. Beidaihe, Qinhuangdao and Nandaihe in Huludao, which you visited before, are basically seaside. Yue Tuo Island belongs to the ocean, with less garbage and clean sea water, but it is not as clean as those tropical islands at low latitudes, but it is much cleaner than the seaside.
If in doubt, you can directly ask the official designated center of Laoting Tourism.
Laoting travel information consultation service center
Address: No.88 Yong 'an South Road, Laoting County (near Dadong Shopping Plaza)