It's a boy walking on the beach. His name is Hamchuck. And, if you ever saw a map of Vietnam, especially south Vietnam where the US soldiers were stationed, there are beaches on both sides, the east and west. This isn't a slip up.
That a sunset in Vietnam would be viewed over the ocean is a common criticism of the movie. I never thought about it when I saw the movie, and was confused, and remain confused -- I always thought it was a sunrise, not a sunset.
I always thought the bigger flaw is that the Green Berets were fighting in the middle of what looked like evergreen trees with pine needles on the ground. Didn't seem like Vietnam to me (never been there however).