What is the temperature of the see in Dalian on the beginning of September?

The average temperature during that time would be around 20 C. It is very pleasant to enjoy yourself at the seashore then. I suggest you take along T-shirts, shorts, skirts, sandals, sunglasses, hats and suntan oil with you if you intent to the beach.
Golden Stone Beach is suggested and it would bring you a lot of fun with the soft sand and some sandy sculptures.
As for the vaccination, I advise you to ask your doctor for help. They will tell you what kind of vaccination you should take.